Merge pull request #889 from gitter-badger/gitter-badge

Add a Gitter chat badge to README.md

README.md

 [Grafana](http://grafana.org) [![Build Status](https://api.travis-ci.org/grafana/grafana.svg)](https://travis-ci.org/grafana/grafana) [![Coverage Status](https://coveralls.io/repos/grafana/grafana/badge.png)](https://coveralls.io/r/grafana/grafana)
 ================
+[![Gitter](https://badges.gitter.im/Join Chat.svg)](https://gitter.im/grafana/grafana?utm_source=badge&utm_medium=badge&utm_campaign=pr-badge&utm_content=badge)
 [Website](http://grafana.org) |
 [Twitter](http://twitter.com/grafana) |
 [IRC](http://webchat.freenode.net/?channels=grafana) |