add slack community badge

Author: richleland

README.md

@@ -5,7 +5,7 @@
 # SparkPost PHP Library
 
 [![Travis CI](https://travis-ci.org/SparkPost/php-sparkpost.svg?branch=master)](https://travis-ci.org/SparkPost/php-sparkpost)
-[![Coverage Status](https://coveralls.io/repos/SparkPost/php-sparkpost/badge.svg?branch=master&service=github)](https://coveralls.io/github/SparkPost/php-sparkpost?branch=master)
+[![Coverage Status](https://coveralls.io/repos/SparkPost/php-sparkpost/badge.svg?branch=master&service=github)](https://coveralls.io/github/SparkPost/php-sparkpost?branch=master) [![Slack Status](http://slack.sparkpost.com/badge.svg)](http://slack.sparkpost.com)
 
 The official PHP binding for your favorite SparkPost APIs!