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/dccuchile/bert-base-spanish-wwm-uncased/README.md

README.md

@@ -0,0 +1,3 @@
+---
+language: es
+---