Add coverage badge

Author: robinst

README.md

@@ -24,6 +24,8 @@ for other build systems):
 ```
 
 [![Build status](https://travis-ci.org/robinst/autolink-java.svg?branch=master)](https://travis-ci.org/robinst/autolink-java)
+[![Coverage status](https://coveralls.io/repos/github/robinst/autolink-java/badge.svg?branch=master)](https://coveralls.io/github/robinst/autolink-java?branch=master)
+
 
 Usage
 -----