Add a Gitter chat badge to README.md (sjtug#281)

README.md

@@ -2,7 +2,8 @@
 
 [![Build Status](https://travis-ci.org/sjtug/SJTUThesis.svg?branch=master)](https://travis-ci.org/sjtug/SJTUThesis)
 [![GitHub Release](https://img.shields.io/github/release/sjtug/SJTUThesis.svg)](https://github.com/sjtug/SJTUThesis/releases)
-[![GitHub Release Date](https://img.shields.io/github/release-date/sjtug/SJTUThesis.svg)]()
+[![GitHub Release Date](https://img.shields.io/github/release-date/sjtug/SJTUThesis.svg)](https://github.com/sjtug/SJTUThesis/releases)
+[![Join the chat at https://gitter.im/sjtug/SJTUThesis](https://badges.gitter.im/sjtug/SJTUThesis.svg)](https://gitter.im/sjtug/SJTUThesis?utm_source=badge&utm_medium=badge&utm_campaign=pr-badge&utm_content=badge)
 
 这是为撰写上海交通大学学士、硕士或博士论文而准备的 XeLaTeX 模板，非官方出品。生成的学位论文文件参见 [README.pdf][README]，详细使用说明参见 [SJTUThesis Wiki](https://github.com/sjtug/SJTUThesis/wiki)。