[readme] add coverage badge

Author: hazendaz

README.md

@@ -2,6 +2,7 @@ MyBatis Velocity Support
 ========================
 
 [![Build Status](https://travis-ci.org/mybatis/velocity-scripting.svg?branch=master)](https://travis-ci.org/mybatis/velocity-scripting)
+[![Coverage Status](https://coveralls.io/repos/mybatis/velocity-scripting/badge.svg?branch=master&service=github)](https://coveralls.io/github/mybatis/velocity-scripting?branch=master)
 [![Dependency Status](https://www.versioneye.com/user/projects/560f3b3b5a262f001a000a20/badge.svg?style=flat)](https://www.versioneye.com/user/projects/560f3b3b5a262f001a000a20)
 [![Maven central](https://maven-badges.herokuapp.com/maven-central/org.mybatis.scripting/mybatis-velocity/badge.svg)](https://maven-badges.herokuapp.com/maven-central/org.mybatis.scripting/mybatis-velocity)