Add Dependabot configuration file

...in order to update the GitHub Actions package versions.

I noted that the versions of `actions/checkout` in use were out of date. Didn't want to leave it unautomated.

.github/dependabot.yml

@@ -0,0 +1,8 @@
+version: 2
+updates:
+
+  # Maintain dependencies for GitHub Actions
+  - package-ecosystem: "github-actions"
+    directory: "/"
+    schedule:
+      interval: "weekly"