Add dependabot.yml for tracking github action updates

Use the special branch "workflows" for this purpose. It contains
only files related to GitHub workflows.

Signed-off-by: Martin Wilck <mwilck@suse.com>

Author: mwilck

dependabot.yml

@@ -0,0 +1,10 @@
+version: 2
+updates:
+  # Enable version updates for GitHub Actions
+  - package-ecosystem: "github-actions"
+    directory: "/"
+    schedule:
+      interval: "weekly"
+    target-branch: "workflows"
+    commit-message:
+      prefix: "GitHub workflows [dependabot]: "