Adding docs badge (#265)

Author: aliabbasrizvi

README.md

@@ -4,6 +4,7 @@ Optimizely Python SDK
 [![PyPI version](https://badge.fury.io/py/optimizely-sdk.svg)](https://pypi.org/project/optimizely-sdk)
 [![Build Status](https://travis-ci.org/optimizely/python-sdk.svg?branch=master)](https://travis-ci.org/optimizely/python-sdk)
 [![Coverage Status](https://coveralls.io/repos/github/optimizely/python-sdk/badge.svg)](https://coveralls.io/github/optimizely/python-sdk)
+[![Documentation Status](https://readthedocs.org/projects/optimizely-python-sdk/badge/?version=latest)](https://optimizely-python-sdk.readthedocs.io/en/latest/?badge=latest)
 [![Apache 2.0](https://img.shields.io/badge/License-Apache%202.0-blue.svg)](http://www.apache.org/licenses/LICENSE-2.0)
 
 This repository houses the official Python SDK for use with Optimizely