Added Gitter badge

Author: gitter-badger

README.md

@@ -1,5 +1,7 @@
 # Git for Windows
 
+[![Join the chat at https://gitter.im/git-for-windows/git](https://badges.gitter.im/Join%20Chat.svg)](https://gitter.im/git-for-windows/git?utm_source=badge&utm_medium=badge&utm_campaign=pr-badge&utm_content=badge)
+
 This is the source code of [Git for Windows](http://git-for-windows.github.io/),
 forked from [Git](http://git-scm.com/).