Add a link to Discord

Author: bbatsov

README.md

@@ -2,6 +2,7 @@
 [![MELPA][melpa-badge]][melpa-package]
 [![MELPA Stable][melpa-stable-badge]][melpa-stable-package]
 [![circleci][badge-circleci]][circleci]
+[![Discord](https://img.shields.io/badge/chat-on%20discord-7289da.svg?sanitize=true)](https://discord.com/invite/nFPpynQPME)
 
 # Clojure Mode