add slack to readme

Author: leebyron

README.md

@@ -6,6 +6,7 @@ on complex application data models.
 
 [![Build Status](https://travis-ci.org/graphql/graphql-js.svg)](https://travis-ci.org/graphql/graphql-js)
 [![Coverage Status](https://coveralls.io/repos/graphql/graphql-js/badge.svg?branch=master)](https://coveralls.io/r/graphql/graphql-js?branch=master)
+[![Public Slack Discussion](https://graphql-slack.herokuapp.com/badge.svg)](https://graphql-slack.herokuapp.com/)
 
 ## Technical Preview Contents