message: docs: add a note to RELEASING about Slack Channel
diff: @@ -22,6 +22,11 @@ under the License.
  You'll probably want to run these commands manually and understand what
  they do prior to doing so.
  +For coordinating on releases, on more operational topics that require more
  +synchronous communications, we tend to use the `#apache-releases` channel
  +on the Superset Slack. People crafting releases and those interested in
  +partaking in the process should join the channel.
  +
  ## Release setup
  First you need to setup a few things. This is a one-off and doesn't
  