Add Dependabot for GitHub Actions (#488)

This PR adds Dependabot for GitHub Actions as described here -
https://docs.github.com/en/code-security/dependabot/working-with-dependabot/keeping-your-actions-up-to-date-with-dependabot

As the checkout action is currently out of date, Dependabot will open a
PR to update it if and when this PR is merged.

## All PRs:

* Has tests - N/A as this is a CI configuration issue
* Documentation updated - N/A as this is a CI configuration issue

Author: lgarron

.github/dependabot.yml

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