Added docs badge

Author: MSeal

README.md

@@ -1,5 +1,6 @@
 [![Binder](https://mybinder.org/badge_logo.svg)](https://mybinder.org/v2/gh/jupyter/nbclient/master?filepath=binder%2Frun_nbclient.ipynb)
 [![Travis Build Status](https://travis-ci.org/jupyter/nbclient.svg?branch=master)](https://travis-ci.org/jupyter/nbclient)
+[![Documentation Status](https://readthedocs.org/projects/nbclient/badge/?version=latest)](https://nbclient.readthedocs.io/en/latest/?badge=latest)
 [![image](https://codecov.io/github/jupyter/nbclient/coverage.svg?branch=master)](https://codecov.io/github/jupyter/nbclient?branch=master)
 [![Python 3.6](https://img.shields.io/badge/python-3.6-blue.svg)](https://www.python.org/downloads/release/python-360/)
 [![Python 3.7](https://img.shields.io/badge/python-3.7-blue.svg)](https://www.python.org/downloads/release/python-370/)