Add a Gitter chat badge to README.md

### zimme/meteor-bootstrap-growl now has a Chat Room on Gitter

@zimme has just created a chat room. You can visit it here: https://gitter.im/zimme/meteor-bootstrap-growl.

This pull-request adds this badge to your README.md:

[![Gitter](https://badges.gitter.im/Join Chat.svg)](https://gitter.im/zimme/meteor-bootstrap-growl?utm_source=badge&utm_medium=badge&utm_campaign=pr-badge)

Happy chatting.