mention Slack

Vladimir Kotal · web-flow · commit 2af09da302d2 · 2018-10-23T22:22:58.000+02:00
diff --git a/README.md b/README.md
@@ -51,6 +51,8 @@ Feel free to participate in discussion on the mailing lists:
 
 To subscribe, send email to `<mailing_list_name>-subscribe@yahoogroups.com`
 
+There are also Slack channels on https://opengrok.slack.com/
+
 ## 6. Demo
 
 Visit <http://demo.opengrok.org>
