How to add new pre-trained model pytorch-transformers
## ❓ Questions & Help

Pytorch-transformers is a great library. I like that it does one thing, give access to the pre-trained SOTA models for NLP. 
I and my team we want to help and start contributing: 
- at first, want to add Polish BERT model like https://github.com/huggingface/pytorch-transformers/pull/688

But we do not know how to do this :( 
Is there any guide or procedure that shows what should be changed in order to add a new model?
We would be grateful if someone guides us.
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