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/aodiniz/bert_uncased_L-2_H-512_A-8_cord19-200616/README.md

README.md

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+# BERT L-2 H-512 fine-tuned on MLM (CORD-19 2020/06/16)
+
+BERT model with [2 Transformer layers and hidden embedding of size 512](https://huggingface.co/google/bert_uncased_L-2_H-512_A-8), referenced in [Well-Read Students Learn Better: On the Importance of Pre-training Compact Models](https://arxiv.org/abs/1908.08962), fine-tuned for MLM on CORD-19 dataset (as released on 2020/06/16).
+
+## Training the model
+
+```bash
+python run_language_modeling.py
+    --model_type bert
+    --model_name_or_path google/bert_uncased_L-2_H-512_A-8
+    --do_train
+    --train_data_file {cord19-200616-dataset}
+    --mlm
+    --mlm_probability 0.2
+    --line_by_line
+    --block_size 512
+    --per_device_train_batch_size 20
+    --learning_rate 3e-5
+    --num_train_epochs 2
+    --output_dir bert_uncased_L-2_H-512_A-8_cord19-200616