Migrate model card from transformers-repo

Read announcement at https://discuss.huggingface.co/t/announcement-all-model-cards-will-be-migrated-to-hf-co-model-repos/2755
Original file history: https://github.com/huggingface/transformers/commits/master/model_cards/elgeish/cs224n-squad2.0-roberta-base/README.md

README.md

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+## CS224n SQuAD2.0 Project Dataset
+The goal of this model is to save CS224n students GPU time when establishing
+baselines to beat for the [Default Final Project](http://web.stanford.edu/class/cs224n/project/default-final-project-handout.pdf).
+The training set used to fine-tune this model is the same as
+the [official one](https://rajpurkar.github.io/SQuAD-explorer/); however,
+evaluation and model selection were performed using roughly half of the official
+dev set, 6078 examples, picked at random. The data files can be found at
+<https://github.com/elgeish/squad/tree/master/data> — this is the Winter 2020
+version. Given that the official SQuAD2.0 dev set contains the project's test
+set, students must make sure not to use the official SQuAD2.0 dev set in any way
+— including the use of models fine-tuned on the official SQuAD2.0, since they
+used the official SQuAD2.0 dev set for model selection.
+
+## Results
+```json
+{
+  "exact": 75.32082922013821,
+  "f1": 78.66699523704254,
+  "total": 6078,
+  "HasAns_exact": 74.84536082474227,
+  "HasAns_f1": 81.83436324767868,
+  "HasAns_total": 2910,
+  "NoAns_exact": 75.75757575757575,
+  "NoAns_f1": 75.75757575757575,
+  "NoAns_total": 3168,
+  "best_exact": 75.32082922013821,
+  "best_exact_thresh": 0.0,
+  "best_f1": 78.66699523704266,
+  "best_f1_thresh": 0.0
+}
+```
+
+## Notable Arguments
+```json
+{
+  "do_lower_case": true,
+  "doc_stride": 128,
+  "fp16": false,
+  "fp16_opt_level": "O1",
+  "gradient_accumulation_steps": 24,
+  "learning_rate": 3e-05,
+  "max_answer_length": 30,
+  "max_grad_norm": 1,
+  "max_query_length": 64,
+  "max_seq_length": 384,
+  "model_name_or_path": "roberta-base",
+  "model_type": "roberta",
+  "num_train_epochs": 4,
+  "per_gpu_train_batch_size": 16,
+  "save_steps": 5000,
+  "seed": 42,
+  "train_batch_size": 16,
+  "version_2_with_negative": true,
+  "warmup_steps": 0,
+  "weight_decay": 0
+}
+```
+
+## Environment Setup
+```json
+{
+  "transformers": "2.5.1",
+  "pytorch": "1.4.0=py3.6_cuda10.1.243_cudnn7.6.3_0",
+  "python": "3.6.5=hc3d631a_2",
+  "os": "Linux 4.15.0-1060-aws #62-Ubuntu SMP Tue Feb 11 21:23:22 UTC 2020 x86_64 x86_64 x86_64 GNU/Linux",
+  "gpu": "Tesla V100-SXM2-16GB"
+}
+```
+
+## How to Cite
+```BibTeX
+@misc{elgeish2020gestalt,
+  title={Gestalt: a Stacking Ensemble for SQuAD2.0},
+  author={Mohamed El-Geish},
+  journal={arXiv e-prints},
+  archivePrefix={arXiv},
+  eprint={2004.07067},
+  year={2020},
+}
+```
+
+## Related Models
+* [elgeish/cs224n-squad2.0-albert-base-v2](https://huggingface.co/elgeish/cs224n-squad2.0-albert-base-v2)
+* [elgeish/cs224n-squad2.0-albert-large-v2](https://huggingface.co/elgeish/cs224n-squad2.0-albert-large-v2)
+* [elgeish/cs224n-squad2.0-albert-xxlarge-v1](https://huggingface.co/elgeish/cs224n-squad2.0-albert-xxlarge-v1)
+* [elgeish/cs224n-squad2.0-distilbert-base-uncased](https://huggingface.co/elgeish/cs224n-squad2.0-distilbert-base-uncased)