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/xlm-roberta-base-README.md

README.md

@@ -0,0 +1,10 @@
+---
+tags:
+- exbert
+
+license: mit
+---
+
+<a href="https://huggingface.co/exbert/?model=xlm-roberta-base">
+	<img width="300px" src="https://cdn-media.huggingface.co/exbert/button.png">
+</a>