bot: add bot message when pushing on a release branch

Normally, only the mergify bot should send out PR to the release
branches. All contributions should generally go in master first then be
backported. Let's warn to avoid merging PR send against a release
branch.

Signed-off-by: Sébastien Han <seb@redhat.com>

.mergify.yml

@@ -7,6 +7,14 @@ pull_request_rules:
       comment:
         message: This pull request has merge conflicts that must be resolved before it can be merged. @{{author}} please rebase it. https://rook.io/docs/rook/latest/development-flow.html#updating-your-fork
 
+  - name: ping author on direct push to release branch
+    conditions:
+      - base~=^release-
+      - author!=mergify[bot]
+    actions:
+      comment:
+        message: Hi @{{author}}, this pull request was opened against a release branch, is it expected? Normally patches should go in the master branch first and then be backported to release branches.
+
   # automerge on master only under certain strict conditions
   - name: automerge merge master with specific label and approvals with code change
     conditions: