Update ADD_BIG_BIRD.md

templates/adding_a_new_model/open_model_proposals/ADD_BIG_BIRD.md

@@ -3,10 +3,12 @@ How to add BigBird to 🤗 Transformers?
 
 Mentor: [Patrick](https://github.com/patrickvonplaten)
 
-Begin: 19.02.2020
+Begin: 12.02.2020
 
 Estimated End: 19.03.2020
 
+Contributor: [Vasudev](https://github.com/vasudevgupta7)
+
 Adding a new model is often difficult and requires an in-depth knowledge
 of the 🤗 Transformers library and ideally also of the model's original
 repository. At Hugging Face, we are trying to empower the community more