HBASE-28737 Add the slack channel related information in README.md (#6087)

Signed-off-by: Yi Mei <meiyi@apache.org>
(cherry picked from commit 38e5983)

Author: Apache9

README.md

@@ -28,6 +28,8 @@ The latest HBase can be downloaded from the [download page](https://hbase.apache
 
 We use mailing lists to send notice and discuss. The mailing lists and archives are listed [here](http://hbase.apache.org/mail-lists.html)
 
+We use the #hbase channel on the official [ASF Slack Workspace](https://the-asf.slack.com/) for real time questions and discussions. Please mail dev@hbase.apache.org to request an invite.
+
 # How to Contribute
 The source code can be found at https://hbase.apache.org/source-repository.html