add travis-ci in readme

Change-Id: I026a581d60c56f4cb58b77a51eb274a2c16e5397

Author: py8765

README.md

@@ -4,6 +4,8 @@ Pomelo is a fast, scalable game server framework for [node.js](http://nodejs.org
 It provides the basic development framework and many related components, including libraries and tools.
 Pomelo is also suitable for real-time web applications; its distributed architecture makes pomelo scale better than other real-time web frameworks.
 
+[![Build Status](https://travis-ci.org/NetEase/pomelo.svg?branch=master)](https://travis-ci.org/NetEase/pomelo)
+
  * Homepage: <http://pomelo.netease.com/>
  * Mailing list: <https://groups.google.com/group/pomelo>
  * Documentation: <http://github.com/NetEase/pomelo>