Add gitter badge (closes #157)

README.md

@@ -1,6 +1,9 @@
 # Snowplow-Mini
 
-[![Build Status][travis-image]][travis] [![Release][release-image]][releases] [![License][license-image]][license]
+[![Join the chat at https://gitter.im/snowplow/snowplow-mini](https://badges.gitter.im/snowplow/snowplow-mini.svg)](https://gitter.im/snowplow/snowplow-mini?utm_source=badge&utm_medium=badge&utm_campaign=pr-badge&utm_content=badge)
+[![Build Status][travis-image]][travis]
+[![Release][release-image]][releases]
+[![License][license-image]][license]
 
 An easily-deployable, single instance version of Snowplow that serves three use cases: