added dependabot support for github actions

Author: enekofb

.github/dependabot.yml

@@ -25,3 +25,17 @@ updates:
       - "weaveworks/tangerine"
     # Only do security updates not version updates.
     open-pull-requests-limit: 0
+
+  # maintain dependencies for github actions
+  - package-ecosystem: "github-actions"
+    directory: "/"
+    schedule:
+      interval: "weekly"
+    reviewers:
+      - "weaveworks/timber-wolf"
+      - "weaveworks/pesto"
+      - "weaveworks/wild-watermelon"
+      - "weaveworks/tangerine"
+    # Only do security updates not version updates.
+    open-pull-requests-limit: 0
+