[README] link to Write With Transformer
Tomorrow we'll release a new version of Write With Transformer that's gonna let you:
- experiment with different models (gpt2, xlnet) and/or model checkpoints (example for gpt2: small, large, arxiv)
- Share links to your documents.

With those two changes transfomer.huggingface.co is graduating to being an official demo for `pytorch-transformers`'s text generation capabilities.