Adding docs badge

Author: codertimo

README.md

@@ -6,6 +6,7 @@
 [![CircleCI](https://circleci.com/gh/codertimo/BERT-pytorch.svg?style=shield)](https://circleci.com/gh/kor2vec/kor2vec)
 [![PyPI](https://img.shields.io/pypi/v/bert-pytorch.svg)](https://pypi.org/project/bert_pytorch/)
 [![PyPI - Status](https://img.shields.io/pypi/status/bert-pytorch.svg)](https://pypi.org/project/bert_pytorch/)
+[![Documentation Status](https://readthedocs.org/projects/bert-pytorch/badge/?version=latest)](https://bert-pytorch.readthedocs.io/en/latest/?badge=latest)
 
 Pytorch implementation of Google AI's 2018 BERT, with simple annotation