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/julien-c/dummy-unknown/README.md

README.md

@@ -1,9 +1,15 @@
+---
+tags:
+- ci
+---
+
+## Dummy model used for unit testing and CI
+
 
 ```python
 import json
 import os
-from transformers.configuration_roberta import RobertaConfig
-from transformers import RobertaForMaskedLM, TFRobertaForMaskedLM
+from transformers import RobertaConfig, RobertaForMaskedLM, TFRobertaForMaskedLM
 
 DIRNAME = "./dummy-unknown"