Add a Gitter chat badge to README.md (#422)

Author: gitter-badger

README.md

@@ -2,8 +2,9 @@
 
 > Documentation generator for TypeScript projects.
 
-[![Build Status](https://travis-ci.org/TypeStrong/typedoc.svg?branch=master)](https://travis-ci.org/TypeStrong/typedoc) [![NPM version](https://badge.fury.io/js/typedoc.svg)](http://badge.fury.io/js/typedoc)
-
+[![Build Status](https://travis-ci.org/TypeStrong/typedoc.svg?branch=master)](https://travis-ci.org/TypeStrong/typedoc) 
+[![NPM Version](https://badge.fury.io/js/typedoc.svg)](http://badge.fury.io/js/typedoc)
+[![Chat on Gitter](https://badges.gitter.im/TypeStrong/typedoc.svg)](https://gitter.im/TypeStrong/typedoc?utm_source=badge&utm_medium=badge&utm_campaign=pr-badge&utm_content=badge)
 
 ## Installation