[model_cards] Link to ExBERT visualisation

Hat/tip @bhoov @HendrikStrobelt @sebastianGehrmann

Also cc @srush and @thomwolf

model_cards/bert-base-german-cased-README.md

@@ -1,8 +1,12 @@
 ---
 language: german
 thumbnail: https://static.tildacdn.com/tild6438-3730-4164-b266-613634323466/german_bert.png
+tags:
+- exbert
 ---
 
+[![ExBERT](https://img.shields.io/badge/Visualize%20Attentions-ExBERT-green)](https://huggingface.co/exbert/?model=bert-base-german-cased)
+
 # German BERT
 ![bert_image](https://static.tildacdn.com/tild6438-3730-4164-b266-613634323466/german_bert.png)
 ## Overview