Update README
Author: sgugger
Pull request: yes

This PR reorganizes the README to reflect the current state of transformers a bit more. croll down a bit [here](https://github.com/huggingface/transformers/tree/update_readme) for a preview.

Here is the plan I followed:

**Introduction:** Expand a bit on the first two sentences to explain a bit more generally what this library is about.

The idea to have content aimed at both researchers/NLP experts and data scientists/engineers. Since it gets longer, we could move it after the top contributors badges.

**Online demo:** I think it makes more sense to have this right after the introduction, to show what transformers can do.

**Code sample:** Then we switch to how to do it, showcasing two shorts examples: pipeline and a pretrained model loading/example of use. Let’s not overwhelm a reader with more than that and link to our tutorials for more examples.

**Feature list:** This becomes Why use/not-use transformers but the spirit remains the same.

**Installation:** Keeping this simple and to the point, linking to the documentation for more in-depth content.

**Model architectures:** I've kept the full list with papers but I'm not sure it makes sense here as we also have it in the index of our doc. An alternative would be to show this in terms of tasks and give examples for each one of models supporting them (so key names still appear) with links to the docs.

**Learn more:** This is where all the content I removed is linked.

**Citation**


Comments:
# [Codecov](https://codecov.io/gh/huggingface/transformers/pull/7133?src=pr&el=h1) Report
> Merging [#7133](https://codecov.io/gh/huggingface/transformers/pull/7133?src=pr&el=desc) into [master](https://codecov.io/gh/huggingface/transformers/commit/90cde2e938638e64a8696a12b79ee5f52364b162?el=desc) will **increase** coverage by `1.87%`.
> The diff coverage is `n/a`.

[![Impacted file tree graph](https://codecov.io/gh/huggingface/transformers/pull/7133/graphs/tree.svg?width=650&height=150&src=pr&token=9qOlN6Hb1c)](https://codecov.io/gh/huggingface/transformers/pull/7133?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master    #7133      +/-   ##
==========================================
+ Coverage   79.62%   81.49%   +1.87%     
==========================================
  Files         168      168              
  Lines       32284    32284              
==========================================
+ Hits        25706    26310     +604     
+ Misses       6578     5974     -604     
```


| [Impacted Files](https://codecov.io/gh/huggingface/transformers/pull/7133?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [src/transformers/tokenization\_pegasus.py](https://codecov.io/gh/huggingface/transformers/pull/7133/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy90b2tlbml6YXRpb25fcGVnYXN1cy5weQ==) | `46.03% <0.00%> (-49.21%)` | :arrow_down: |
| [src/transformers/tokenization\_funnel.py](https://codecov.io/gh/huggingface/transformers/pull/7133/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy90b2tlbml6YXRpb25fZnVubmVsLnB5) | `62.79% <0.00%> (-34.89%)` | :arrow_down: |
| [src/transformers/modeling\_tf\_distilbert.py](https://codecov.io/gh/huggingface/transformers/pull/7133/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ190Zl9kaXN0aWxiZXJ0LnB5) | `64.55% <0.00%> (-34.28%)` | :arrow_down: |
| [src/transformers/tokenization\_albert.py](https://codecov.io/gh/huggingface/transformers/pull/7133/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy90b2tlbml6YXRpb25fYWxiZXJ0LnB5) | `70.19% <0.00%> (-23.08%)` | :arrow_down: |
| [src/transformers/tokenization\_openai.py](https://codecov.io/gh/huggingface/transformers/pull/7133/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy90b2tlbml6YXRpb25fb3BlbmFpLnB5) | `71.21% <0.00%> (-12.88%)` | :arrow_down: |
| [src/transformers/modeling\_mobilebert.py](https://codecov.io/gh/huggingface/transformers/pull/7133/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ19tb2JpbGViZXJ0LnB5) | `79.21% <0.00%> (-10.25%)` | :arrow_down: |
| [src/transformers/tokenization\_auto.py](https://codecov.io/gh/huggingface/transformers/pull/7133/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy90b2tlbml6YXRpb25fYXV0by5weQ==) | `81.96% <0.00%> (-9.84%)` | :arrow_down: |
| [src/transformers/configuration\_bart.py](https://codecov.io/gh/huggingface/transformers/pull/7133/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9jb25maWd1cmF0aW9uX2JhcnQucHk=) | `90.00% <0.00%> (-4.00%)` | :arrow_down: |
| [src/transformers/generation\_tf\_utils.py](https://codecov.io/gh/huggingface/transformers/pull/7133/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9nZW5lcmF0aW9uX3RmX3V0aWxzLnB5) | `85.46% <0.00%> (-1.26%)` | :arrow_down: |
| [src/transformers/configuration\_utils.py](https://codecov.io/gh/huggingface/transformers/pull/7133/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9jb25maWd1cmF0aW9uX3V0aWxzLnB5) | `96.00% <0.00%> (-0.67%)` | :arrow_down: |
| ... and [6 more](https://codecov.io/gh/huggingface/transformers/pull/7133/diff?src=pr&el=tree-more) | |

------

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> Powered by [Codecov](https://codecov.io/gh/huggingface/transformers/pull/7133?src=pr&el=footer). Last update [90cde2e...8faad23](https://codecov.io/gh/huggingface/transformers/pull/7133?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).