mts: add example rule to rule file

Signed-off-by: Chenxiong Qi <cqi@redhat.com>

roles/openshift-apps/message-tagging-service/files/mts-rules.yml

@@ -1,3 +1,15 @@
 ---
 # Please add rules to tag module build.
 # Refer to https://pagure.io/modularity/blob/master/f/drafts/module-tagging-service/format.md
+
+# This is an example rule. Remove it and add new one when MTS is enabled in prod.
+- id: Exmaple rule
+  description: Apply gating tag to module build
+  type: module
+  rule:
+    dependencies:
+      buildrequires:
+        platform: '^f(?P<platform>\d+)$'
+      requires:
+        platform: '^f\d+$'
+  destinations: 'some-tag-f\g<platform>-gate'