Start Dependabot for GitHub Actions

References:
* https://docs.github.com/en/code-security/dependabot/working-with-dependabot/dependabot-options-reference

Signed-off-by: Alex Nelson <alexander.nelson@nist.gov>

Author: ajnelson-nist

.github/dependabot.yml

@@ -0,0 +1,7 @@
+version: 2
+updates:
+  - package-ecosystem: "github-actions"
+    directory: "/"
+    schedule:
+      interval: "weekly"
+    # target-branch: "develop"