adding code coverage badge

Author: aliabbasrizvi

.coveragerc

@@ -1,3 +1,3 @@
 [run]
 omit =
-  optimizely/lib/pymmh3*
+  optimizely/lib/pymmh3*

README.md

@@ -1,5 +1,6 @@
 #Optimizely Python 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)
 [![Apache 2.0](https://img.shields.io/github/license/nebula-plugins/gradle-extra-configurations-plugin.svg)](http://www.apache.org/licenses/LICENSE-2.0)
 
 This repository houses the Python SDK for Optimizely's server-side testing product, which is currently in private beta.