Add gitter badge

README.md

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 [![Build Status](https://circleci.com/gh/documentationjs/documentation.svg?style=svg)](https://circleci.com/gh/documentationjs/documentation) [![Coverage Status](https://coveralls.io/repos/documentationjs/documentation/badge.svg?branch=master)](https://coveralls.io/r/documentationjs/documentation?branch=master)
 [![npm version](https://badge.fury.io/js/documentation.svg)](http://badge.fury.io/js/documentation)
+[![Gitter](https://badges.gitter.im/Join%20Chat.svg)](https://gitter.im/documentationjs/documentation?utm_source=badge&utm_medium=badge&utm_campaign=pr-badge)
 
 A **documentation generation system** that's
 _beautiful_ by default, _flexible_ across formats and styles, and