message: Add Slack channel
diff: @@ -23,6 +23,7 @@ mod_pagespeed is built on PageSpeed Optimization Libraries, deployed across 100,
  | Try it | [modpagespeed.com](https://modpagespeed.com) |
  |--- |--- |
  +| Slack | https://the-asf.slack.com/archives/CJTG9RH9U |
  | Read about it |https://developers.google.com/speed/pagespeed/module |
  | Download it | https://modpagespeed.com/doc/download |
  | Check announcements |https://groups.google.com/group/mod-pagespeed-announce |
  