Add coverage badge to README.

Author: craigcitro

README.md

@@ -1,5 +1,7 @@
 [![Build Status](https://travis-ci.org/google/oauth2client.svg?branch=master)](https://travis-ci.org/google/oauth2client)
 
+[![Coverage Status](https://img.shields.io/coveralls/google/oauth2client.svg)](https://coveralls.io/r/google/oauth2client?branch=master)
+
 This is a client library for accessing resources protected by OAuth 2.0.
 
 [Full documentation](http://google.github.io/oauth2client/)