Add slackin badge.

README.md

@@ -1,6 +1,8 @@
 [![stratux version](https://img.shields.io/github/tag/cyoung/stratux.svg?style=flat&label=stratux)](https://github.com/cyoung/stratux/releases)
 [![Build Status](http://circleci-badges-max.herokuapp.com/img/cyoung/stratux/master?token=:circle-ci-token)](https://circleci.com/gh/cyoung/stratux/tree/master)
 [![BSD3 License](http://img.shields.io/badge/license-BSD3-brightgreen.svg)](https://tldrlegal.com/license/bsd-3-clause-license-%28revised%29)
+[![Stratux Slack](http://slack.stratux.me:3000/badge.svg)](http://slack.stratux.me/)
+
 # stratux
 RTL-SDR UAT tools