Enable Dependabot

Configure Dependabot to automatically detect updates to the actions used in
GitHub Actions (e.g., `actions/checkout`) and create pull requests for them.

Based on experience, checking for updates on a weekly interval should work well.

Author: koic

dependabot.yml

@@ -0,0 +1,6 @@
+version: 2
+updates:
+  - package-ecosystem: 'github-actions'
+    directory: '/'
+    schedule:
+      interval: 'weekly'