doc: add Slack community to support options

PR-URL: #18191
Reviewed-By: Rich Trott <rtrott@gmail.com>
Reviewed-By: Benjamin Gruenbaum <benjamingr@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Trivikram Kamat <trivikr.dev@gmail.com>

README.md

@@ -55,6 +55,8 @@ search these unofficial resources:
 * [Questions tagged 'node.js' on StackOverflow][]
 * [#node.js channel on chat.freenode.net][]. See <http://nodeirc.info/> for more
   information.
+* [Node.js Slack Community](https://node-js.slack.com/): Visit
+  [nodeslackers.com](http://www.nodeslackers.com/) to register.
 
 GitHub issues are meant for tracking enhancements and bugs, not general support.