add coverage badge

README.md

@@ -9,6 +9,7 @@ Wechaty is a Bot-Enable Framework/Library for Personal Account of Wechat.
 [![Join the chat at https://gitter.im/zixia/wechaty](https://badges.gitter.im/zixia/wechaty.svg)](https://gitter.im/zixia/wechaty?utm_source=badge&utm_medium=badge&utm_campaign=pr-badge&utm_content=badge)
 [![node](https://img.shields.io/node/v/wechaty.svg?maxAge=2592000)](https://nodejs.org/)
 [![Repo Size](https://reposs.herokuapp.com/?path=zixia/wechaty)]()
+[![Coverage Status](https://coveralls.io/repos/github/zixia/wechaty/badge.svg?branch=master)](https://coveralls.io/github/zixia/wechaty?branch=master)
 [![npm version](https://badge.fury.io/js/wechaty.svg)](https://badge.fury.io/js/wechaty)
 [![Downloads][downloads-image]][downloads-url]