added gitter.im chat link to readme

I've added a gitter.im chat link to the project.
Before the pull request is accepted following points need to be considered:

 1. Do we want to provide a chat? If we do, then it should be 'maintained' to a certain extend.
 2. Do we want to use gitter.im (alternatives?)?
 3. Do we want an org-wide chat or a per-project chat (gitter.im/AndroidBootstrap vs gitter.im/AndroidBootstrap/android-bootstrap) This pull request uses org-wide