Per @evan-palmer, trying out mergify to maintain ROS2 branches

.github/mergify.yml

@@ -0,0 +1,26 @@
+pull_request_rules:
+  - name: backport to iron at reviewers discretion
+    conditions:
+      - base=main
+      - "label=backport-iron"
+    actions:
+      backport:
+        branches:
+          - iron
+
+  - name: ask to resolve conflict
+    conditions:
+      - conflict
+      - author!=mergify[bot]
+    actions:
+        comment:
+          message: This pull request is in conflict. Could you fix it @{{author}}?
+
+  - name: development targets main branch
+    conditions:
+      - base!=main
+      - author!=mergify[bot]
+    actions:
+        comment:
+          message: |
+            Please target the `main` branch for development, we will backport the changes to {{base}} for you if approved and if they don't break API.