model_cards: we can now tag datasets

see corresponding model pages to see how it's rendered

model_cards/canwenxu/BERT-of-Theseus-MNLI/README.md

 ---
 thumbnail: https://raw.githubusercontent.com/JetRunner/BERT-of-Theseus/master/bert-of-theseus.png
+datasets:
+- multi_nli
 ---
 
 # BERT-of-Theseus

model_cards/deepset/roberta-base-squad2/README.md

+---
+datasets:
+- squad_v2
+---
+
 # roberta-base for QA 
 
 ## Overview