update documentation

README-en_US.md

-# 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)
+# 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) [![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)
 
 An enterprise-class UI design language and React-based implementation.