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/cambridgeltl/BioRedditBERT-uncased/README.md

README.md

@@ -1,6 +1,6 @@
 ---
 language: 
-- EN
+- en
 tags:
 - BioNLP
 - social_media