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/VictorSanh/roberta-base-finetuned-yelp-polarity/README.md

README.md

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+---
+language: en
+datasets:
+- yelp_polarity
+---
+
+# RoBERTa-base-finetuned-yelp-polarity
+
+This is a [RoBERTa-base](https://huggingface.co/roberta-base) checkpoint fine-tuned on binary sentiment classifcation from [Yelp polarity](https://huggingface.co/nlp/viewer/?dataset=yelp_polarity).
+It gets **98.08%** accuracy on the test set.
+
+## Hyper-parameters
+
+We used the following hyper-parameters to train the model on one GPU:
+```python
+num_train_epochs            = 2.0
+learning_rate               = 1e-05
+weight_decay                = 0.0
+adam_epsilon                = 1e-08
+max_grad_norm               = 1.0
+per_device_train_batch_size = 32
+gradient_accumulation_steps = 1
+warmup_steps                = 3500
+seed                        = 42
+```