Updated slack badge to discord badge and added a link to the #Redux channel. (#954)

* Updated slack badge to discord badge.

* Update README.md

Author: elongl

README.md

@@ -6,7 +6,7 @@ Performant and flexible.
 
 [![build status](https://img.shields.io/travis/reduxjs/react-redux/master.svg?style=flat-square)](https://travis-ci.org/reduxjs/react-redux) [![npm version](https://img.shields.io/npm/v/react-redux.svg?style=flat-square)](https://www.npmjs.com/package/react-redux)
 [![npm downloads](https://img.shields.io/npm/dm/react-redux.svg?style=flat-square)](https://www.npmjs.com/package/react-redux)
-[![redux channel on slack](https://img.shields.io/badge/slack-redux@reactiflux-61DAFB.svg?style=flat-square)](http://www.reactiflux.com)
+[![redux channel on discord](https://img.shields.io/badge/discord-redux@reactiflux-61DAFB.svg?style=flat-square)](http://www.reactiflux.com)
 
 
 ## Installation