doc(): replace a gitter link with the gitter badge.

Author: hansl

README.md

@@ -1,13 +1,13 @@
 # Material Design for Angular 2
 
 [![Build Status](https://travis-ci.org/angular/material2.svg?branch=master)](https://travis-ci.org/angular/material2)
+[![Gitter](https://badges.gitter.im/angular/material2.svg)](https://gitter.im/angular/material2?utm_source=badge&utm_medium=badge&utm_campaign=pr-badge)
 
 This is the home for the Angular team's Material Design components built on top of Angular 2.
 Things are in very early stages now- check back soon for more progress.
 
 #### Quick links
 [Google group](https://groups.google.com/forum/#!forum/angular-material2),
-[Gitter](https://gitter.im/angular/material2),
 [Contributing](https://github.com/angular/material2/blob/master/CONTRIBUTING.md)
 
 ## The goal of Angular Material