add dependabot config

Dependabot will periodially, once a week, create a PR with updated
common/ submodule if that is needed.
Once a month PRs with bumps of used github actions are to be created
if needed.

.github/dependabot.yml

@@ -0,0 +1,16 @@
+---
+
+version: 2
+updates:
+  - package-ecosystem: gitsubmodule
+    directory: /
+    schedule:
+      interval: weekly
+    labels:
+      - 'ready for review'
+  - package-ecosystem: github-actions
+    directory: /
+    schedule:
+      interval: monthly
+    labels:
+      - 'ready for review'