Add Validation For README

Hi @lhoestq, @yjernite 

This is a simple Readme parser. All classes specific to different sections can inherit `Section` class, and we can define more attributes in each.

Let me know if this is going in the right direction :)

Currently the output looks like this, for `to_dict()` on `FashionMNIST` `README.md`:

```json
{
    "name": "./datasets/fashion_mnist/README.md",
    "attributes": "",
    "subsections": [
        {
            "name": "Dataset Card for FashionMNIST",
            "attributes": "",
            "subsections": [
                {
                    "name": "Table of Contents",
                    "attributes": "- [Dataset Description](#dataset-description)\n  - [Dataset Summary](#dataset-summary)\n  - [Supported Tasks](#supported-tasks-and-leaderboards)\n  - [Languages](#languages)\n- [Dataset Structure](#dataset-structure)\n  - [Data Instances](#data-instances)\n  - [Data Fields](#data-instances)\n  - [Data Splits](#data-instances)\n- [Dataset Creation](#dataset-creation)\n  - [Curation Rationale](#curation-rationale)\n  - [Source Data](#source-data)\n  - [Annotations](#annotations)\n  - [Personal and Sensitive Information](#personal-and-sensitive-information)\n- [Considerations for Using the Data](#considerations-for-using-the-data)\n  - [Social Impact of Dataset](#social-impact-of-dataset)\n  - [Discussion of Biases](#discussion-of-biases)\n  - [Other Known Limitations](#other-known-limitations)\n- [Additional Information](#additional-information)\n  - [Dataset Curators](#dataset-curators)\n  - [Licensing Information](#licensing-information)\n  - [Citation Information](#citation-information)\n  - [Contributions](#contributions)",
                    "subsections": []
                },
                {
                    "name": "Dataset Description",
                    "attributes": "- **Homepage:** [GitHub](https://github.com/zalandoresearch/fashion-mnist)\n- **Repository:** [GitHub](https://github.com/zalandoresearch/fashion-mnist)\n- **Paper:** [arXiv](https://arxiv.org/pdf/1708.07747.pdf)\n- **Leaderboard:**\n- **Point of Contact:**",
                    "subsections": [
                        {
                            "name": "Dataset Summary",
                            "attributes": "Fashion-MNIST is a dataset of Zalando's article images\u2014consisting of a training set of 60,000 examples and a test set of 10,000 examples. Each example is a 28x28 grayscale image, associated with a label from 10 classes. We intend Fashion-MNIST to serve as a direct drop-in replacement for the original MNIST dataset for benchmarking machine learning algorithms. It shares the same image size and structure of training and testing splits.",
                            "subsections": []
                        },
                        {
                            "name": "Supported Tasks and Leaderboards",
                            "attributes": "[More Information Needed]",
                            "subsections": []
                        },
                        {
                            "name": "Languages",
                            "attributes": "[More Information Needed]",
                            "subsections": []
                        }
                    ]
                },
                {
                    "name": "Dataset Structure",
                    "attributes": "",
                    "subsections": [
                        {
                            "name": "Data Instances",
                            "attributes": "A data point comprises an image and its label.",
                            "subsections": []
                        },
                        {
                            "name": "Data Fields",
                            "attributes": "- `image`: a 2d array of integers representing the 28x28 image.\n- `label`: an integer between 0 and 9 representing the classes with the following mapping:\n  | Label | Description |\n  | --- | --- |\n  | 0 | T-shirt/top |\n  | 1 | Trouser |\n  | 2 | Pullover |\n  | 3 | Dress |\n  | 4 | Coat |\n  | 5 | Sandal |\n  | 6 | Shirt |\n  | 7 | Sneaker |\n  | 8 | Bag |\n  | 9 | Ankle boot |",
                            "subsections": []
                        },
                        {
                            "name": "Data Splits",
                            "attributes": "The data is split into training and test set. The training set contains 60,000 images and the test set 10,000 images.",
                            "subsections": []
                        }
                    ]
                },
                {
                    "name": "Dataset Creation",
                    "attributes": "",
                    "subsections": [
                        {
                            "name": "Curation Rationale",
                            "attributes": "**From the arXiv paper:**\nThe original MNIST dataset contains a lot of handwritten digits. Members of the AI/ML/Data Science community love this dataset and use it as a benchmark to validate their algorithms. In fact, MNIST is often the first dataset researchers try. \"If it doesn't work on MNIST, it won't work at all\", they said. \"Well, if it does work on MNIST, it may still fail on others.\"\nHere are some good reasons:\n- MNIST is too easy. Convolutional nets can achieve 99.7% on MNIST. Classic machine learning algorithms can also achieve 97% easily. Check out our side-by-side benchmark for Fashion-MNIST vs. MNIST, and read \"Most pairs of MNIST digits can be distinguished pretty well by just one pixel.\"\n- MNIST is overused. In this April 2017 Twitter thread, Google Brain research scientist and deep learning expert Ian Goodfellow calls for people to move away from MNIST.\n- MNIST can not represent modern CV tasks, as noted in this April 2017 Twitter thread, deep learning expert/Keras author Fran\u00e7ois Chollet.",
                            "subsections": []
                        },
                        {
                            "name": "Source Data",
                            "attributes": "",
                            "subsections": [
                                {
                                    "name": "Initial Data Collection and Normalization",
                                    "attributes": "**From the arXiv paper:**\nFashion-MNIST is based on the assortment on Zalando\u2019s website. Every fashion product on Zalando has a set of pictures shot by professional photographers, demonstrating different aspects of the product, i.e. front and back looks, details, looks with model and in an outfit. The original picture has a light-gray background (hexadecimal color: #fdfdfd) and stored in 762 \u00d7 1000 JPEG format. For efficiently serving different frontend components, the original picture is resampled with multiple resolutions, e.g. large, medium, small, thumbnail and tiny.\nWe use the front look thumbnail images of 70,000 unique products to build Fashion-MNIST. Those products come from different gender groups: men, women, kids and neutral. In particular, whitecolor products are not included in the dataset as they have low contrast to the background. The thumbnails (51 \u00d7 73) are then fed into the following conversion pipeline:\n1. Converting the input to a PNG image.\n2. Trimming any edges that are close to the color of the corner pixels. The \u201ccloseness\u201d is defined by the distance within 5% of the maximum possible intensity in RGB space.\n3. Resizing the longest edge of the image to 28 by subsampling the pixels, i.e. some rows and columns are skipped over.\n4. Sharpening pixels using a Gaussian operator of the radius and standard deviation of 1.0, with increasing effect near outlines.\n5. Extending the shortest edge to 28 and put the image to the center of the canvas.\n6. Negating the intensities of the image.\n7. Converting the image to 8-bit grayscale pixels.",
                                    "subsections": []
                                },
                                {
                                    "name": "Who are the source image producers?",
                                    "attributes": "**From the arXiv paper:**\nEvery fashion product on Zalando has a set of pictures shot by professional photographers, demonstrating different aspects of the product, i.e. front and back looks, details, looks with model and in an outfit.",
                                    "subsections": []
                                }
                            ]
                        },
                        {
                            "name": "Annotations",
                            "attributes": "",
                            "subsections": [
                                {
                                    "name": "Annotation process",
                                    "attributes": "**From the arXiv paper:**\nFor the class labels, they use the silhouette code of the product. The silhouette code is manually labeled by the in-house fashion experts and reviewed by a separate team at Zalando. Each product Zalando is the Europe\u2019s largest online fashion platform. Each product contains only one silhouette code.",
                                    "subsections": []
                                },
                                {
                                    "name": "Who are the annotators?",
                                    "attributes": "**From the arXiv paper:**\nThe silhouette code is manually labeled by the in-house fashion experts and reviewed by a separate team at Zalando.",
                                    "subsections": []
                                }
                            ]
                        },
                        {
                            "name": "Personal and Sensitive Information",
                            "attributes": "[More Information Needed]",
                            "subsections": []
                        }
                    ]
                },
                {
                    "name": "Considerations for Using the Data",
                    "attributes": "",
                    "subsections": [
                        {
                            "name": "Social Impact of Dataset",
                            "attributes": "[More Information Needed]",
                            "subsections": []
                        },
                        {
                            "name": "Discussion of Biases",
                            "attributes": "[More Information Needed]",
                            "subsections": []
                        },
                        {
                            "name": "Other Known Limitations",
                            "attributes": "[More Information Needed]",
                            "subsections": []
                        }
                    ]
                },
                {
                    "name": "Additional Information",
                    "attributes": "",
                    "subsections": [
                        {
                            "name": "Dataset Curators",
                            "attributes": "Han Xiao and Kashif Rasul and Roland Vollgraf",
                            "subsections": []
                        },
                        {
                            "name": "Licensing Information",
                            "attributes": "MIT Licence",
                            "subsections": []
                        },
                        {
                            "name": "Citation Information",
                            "attributes": "@article{DBLP:journals/corr/abs-1708-07747,\n  author    = {Han Xiao and\n               Kashif Rasul and\n               Roland Vollgraf},\n  title     = {Fashion-MNIST: a Novel Image Dataset for Benchmarking Machine Learning\n               Algorithms},\n  journal   = {CoRR},\n  volume    = {abs/1708.07747},\n  year      = {2017},\n  url       = {http://arxiv.org/abs/1708.07747},\n  archivePrefix = {arXiv},\n  eprint    = {1708.07747},\n  timestamp = {Mon, 13 Aug 2018 16:47:27 +0200},\n  biburl    = {https://dblp.org/rec/bib/journals/corr/abs-1708-07747},\n  bibsource = {dblp computer science bibliography, https://dblp.org}\n}",
                            "subsections": []
                        },
                        {
                            "name": "Contributions",
                            "attributes": "Thanks to [@gchhablani](https://github.com/gchablani) for adding this dataset.",
                            "subsections": []
                        }
                    ]
                }
            ]
        }
    ]
}
```

Thanks,
Gunjan

Good start! Here are some proposed next steps:
- We want the Class structure to reflect the template - so the parser know what section titles to expect and when something has gone wrong
- As a result, we don't need to parse the table of contents, since it will always be the same
- For each section/subsection it would be cool to have a variable saying whether it's filled out or not (when it's either empty or has `[More Information Needed]`)
- `attributes` should probably be `text`

@yjernite @lhoestq 

I have added basic validation checking in the class. It works based on a YAML string. The YAML string determines the expected structure and which text is to be checked. The `text` can be true or false showing whether the text has to be checked or not for emptiness. Similarly, each subsection is parsed recursively. I have used print statement currently so that all issues are shown.

Please let me know your thoughts.

I haven't added a variable that keeps a track of whether the text is empty or not but it can be done easliy if required.

This looks like a good start !
Maybe we can use a field named `allow_empty` instead of `text` ?
Also +1 for keeping track of empty texts

Do you think you can have a way to collect all the validation fails of a readme and then raise an error showing all the failures instead of using print ?

Then we can create a `tests/test_dataset_cards.py` test file to make sure all the readmes of the repo are valid !

Hi @lhoestq 

I have added changes accordingly. I prepared a list which stores all the errors and raises them at the end. I'm not sure if there is a better way.

Hi @lhoestq @yjernite 

Please find the output for the existing READMEs here: http://p.ip.fi/2vYU

Thanks,
Gunjan

Hi @lhoestq

I have added some basic tests, also have restructured `ReadMe` class slightly.

There is one print statement currently, I'm not sure how to remove it. Basically, I want to warn but not stop further validation. I can't append to a list because the `error_list` and `warning_list` are both only present in `validate` method, and this print is present in the `parse` method. This is done when someone has repeated a section multiple times.  For e.g.:

```markdown
---
---

# Dataset Card for FashionMNIST
## Dataset Description
## Dataset Description
```

In this case, I check for validation only in the latest entry.

I can also raise an error (ideal case scenario), but still, it is in the `parse`. Should I add `error_lines` and `warning_lines` as instance variables? That would probably solve the issue.

In tests, I'm using a dummy YAML string for structure, we can also make it into a file but I feel that is not a hard requirement. Let me know your thoughts.

I will add tests for `from_readme` as well.

However, I would love to be able to check the exact message in the test when an error is raised. I checked a couple of methods but couldn't get it working. Let me know if you're aware of a way to do that.

Hi @lhoestq 

Thanks for merging. :)
Thanks a lot to you and @yjernite  for guiding me and helping me out.

Yes, I'll also use the next PR for combining the readme and tags validation. ^_^