Added Gitter badge

Author: gitter-badger

README.md

@@ -1,5 +1,7 @@
 maven git commit id plugin
 ==================================
+
+[![Join the chat at https://gitter.im/ktoso/maven-git-commit-id-plugin](https://badges.gitter.im/Join%20Chat.svg)](https://gitter.im/ktoso/maven-git-commit-id-plugin?utm_source=badge&utm_medium=badge&utm_campaign=pr-badge&utm_content=badge)
 [![Build Status](https://secure.travis-ci.org/ktoso/maven-git-commit-id-plugin.svg?branch=master)](http://travis-ci.org/ktoso/maven-git-commit-id-plugin)
 
 git-commit-id-plugin is a plugin quite similar to https://fisheye.codehaus.org/browse/mojo/tags/buildnumber-maven-plugin-1.0-beta-4 for example but as buildnumber at the time when I started this plugin only supported CVS and SVN, something had to be done.