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/NeuML/bert-small-cord19/README.md

README.md

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+# BERT-Small fine-tuned on CORD-19 dataset
+
+[BERT L6_H-512_A-8 model](https://huggingface.co/google/bert_uncased_L-6_H-512_A-8) fine-tuned on the [CORD-19 dataset](https://www.semanticscholar.org/cord19).
+
+## CORD-19 data subset
+The training data for this dataset is stored as a [Kaggle dataset](https://www.kaggle.com/davidmezzetti/cord19-qa?select=cord19.txt). The training
+data is a subset of the full corpus, focusing on high-quality, study-design detected articles.
+
+## Building the model
+
+```bash
+python run_language_modeling.py
+    --model_type bert
+    --model_name_or_path google/bert_uncased_L-6_H-512_A-8
+    --do_train
+    --mlm
+    --line_by_line
+    --block_size 512
+    --train_data_file cord19.txt
+    --per_gpu_train_batch_size 4
+    --learning_rate 3e-5
+    --num_train_epochs 3.0
+    --output_dir bert-small-cord19
+    --save_steps 0
+    --overwrite_output_dir