Add Slack badge to README

README.md

@@ -1,3 +1,4 @@
 Write.as web core
 =================
 [![Build Status](https://travis-ci.org/writeas/web-core.svg)](https://travis-ci.org/writeas/web-core)
+[![Public Slack discussion](http://slack.write.as/badge.svg)](http://slack.write.as/)