Added Gitter badge

README.md

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 # Ant Design [![](https://img.shields.io/travis/ant-design/ant-design.svg?style=flat-square)](https://travis-ci.org/ant-design/ant-design) [![npm package](https://img.shields.io/npm/v/antd.svg?style=flat-square)](https://www.npmjs.org/package/antd) [![Dependency Status](https://david-dm.org/ant-design/ant-design.svg?style=flat-square)](https://david-dm.org/ant-design/ant-design)
 
+[![Join the chat at https://gitter.im/ant-design/ant-design](https://badges.gitter.im/Join%20Chat.svg)](https://gitter.im/ant-design/ant-design?utm_source=badge&utm_medium=badge&utm_campaign=pr-badge&utm_content=badge)
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 一套企业级的前端设计语言和基于 React 的实现。
 
 设计文档和组件实现均在紧密整理和开发中，部分页面可能不完善，预计 8 月份释出正式版本。