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/facebook/bart-large-cnn/README.md

README.md

@@ -0,0 +1,7 @@
+---
+tags:
+- summarization
+
+license: mit
+thumbnail: https://huggingface.co/front/thumbnails/facebook.png
+---