Added Gitter badge

Author: gitter-badger

Diff for: README.md

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 # AngularJS ui-select [![Build Status](https://travis-ci.org/angular-ui/ui-select.svg?branch=master)](https://travis-ci.org/angular-ui/ui-select)
 
+[![Gitter](https://badges.gitter.im/Join%20Chat.svg)](https://gitter.im/angular-ui/ui-select?utm_source=badge&utm_medium=badge&utm_campaign=pr-badge&utm_content=badge)
+
 AngularJS-native version of [Select2](http://ivaynberg.github.io/select2/) and [Selectize](http://brianreavis.github.io/selectize.js/).
 
 - [Demo](http://plnkr.co/edit/a3KlK8dKH3wwiiksDSn2?p=preview)