Added Gitter badge to the README to direct users to the group.

README.md

@@ -5,6 +5,7 @@
 [![Carthage Compatible](https://img.shields.io/badge/Carthage-compatible-4BC51D.svg?style=flat)](https://github.com/Carthage/Carthage)
 [![Platform](https://img.shields.io/cocoapods/p/Alamofire.svg?style=flat)](http://cocoadocs.org/docsets/Alamofire)
 [![Twitter](https://img.shields.io/badge/twitter-@AlamofireSF-blue.svg?style=flat)](http://twitter.com/AlamofireSF)
+[![Gitter](https://badges.gitter.im/Alamofire/Alamofire.svg)](https://gitter.im/Alamofire/Alamofire?utm_source=badge&utm_medium=badge&utm_campaign=pr-badge)
 
 Alamofire is an HTTP networking library written in Swift.