readme: Add Slack badge

Author: maxtruxa

README.md

@@ -4,6 +4,7 @@
 [![npm Downloads][downloads-image]][downloads-url]
 [![Test Status][travis-image]][travis-url]
 [![MIT Licensed][license-image]][license-url]
+[![oauthjs Slack][slack-image]][slack-url]
 
 Complete, compliant and well tested module for implementing an OAuth2 server in [Node.js](https://nodejs.org).
 
@@ -69,4 +70,6 @@ npm test
 [travis-url]: https://travis-ci.org/oauthjs/node-oauth2-server
 [license-image]: https://img.shields.io/badge/license-MIT-blue.svg
 [license-url]: https://raw.githubusercontent.com/oauthjs/node-oauth2-server/master/LICENSE
+[slack-image]: https://img.shields.io/badge/slack-join-E01563.svg
+[slack-url]: https://oauthjs.slack.com