add coverage badge

Author: Evan You

README.md

@@ -1,6 +1,6 @@
 <p align="center"><a href="http://vuejs.org" target="_blank"><img src="http://vuejs.org/images/logo.png"></a></p>
 
-# vue.js [![Build Status](https://travis-ci.org/yyx990803/vue.png?branch=master)](https://travis-ci.org/yyx990803/vue)
+# vue.js [![Build Status](https://travis-ci.org/yyx990803/vue.png?branch=master)](https://travis-ci.org/yyx990803/vue) [![Coverage Status](https://coveralls.io/repos/yyx990803/vue/badge.png)](https://coveralls.io/r/yyx990803/vue)
 
 > Simple, Fast & Composable MVVM for building interative interfaces.