Merge pull request #16591 from ZDroid/patch-1

Add Slack link to `CONTRIBUTING.md`

CONTRIBUTING.md

@@ -19,7 +19,7 @@ and [submitting pull requests](#pull-requests), but please respect the following
 restrictions:
 
 * Please **do not** use the issue tracker for personal support requests.  Stack
-  Overflow ([`twitter-bootstrap-3`](https://stackoverflow.com/questions/tagged/twitter-bootstrap-3) tag), [Slack, or IRC](README.md#community) are better places to get help.
+  Overflow ([`twitter-bootstrap-3`](https://stackoverflow.com/questions/tagged/twitter-bootstrap-3) tag), [Slack](https://bootstrap-slack.herokuapp.com/) or [IRC](README.md#community) are better places to get help.
 
 * Please **do not** derail or troll issues. Keep the discussion on topic and
   respect the opinions of others.