add dependabot.yml (#25)

weekly check for updated github-actions

Author: dennisvang

.github/dependabot.yml

@@ -0,0 +1,7 @@
+# https://docs.github.com/en/code-security/dependabot/working-with-dependabot/dependabot-options-reference
+version: 2
+updates:
+  - package-ecosystem: "github-actions"
+    directory: "/"
+    schedule:
+      interval: "weekly"