[FEATURE] Add a Dependabot action for updating GitHub actions (#452)

With this configuration, GitHub's Dependabot will create PR
to update the versions of the GitHub Actions we are using in
our CI pipeline.

.github/dependabot.yml

@@ -0,0 +1,9 @@
+# https://docs.github.com/github/administering-a-repository/configuration-options-for-dependency-updates
+
+version: 2
+updates:
+  - package-ecosystem: "github-actions"
+    directory: "/"
+    schedule:
+      interval: "daily"
+    milestone: 5

CHANGELOG.md

@@ -6,6 +6,7 @@ This project adheres to [Semantic Versioning](https://semver.org/).
 ## x.y.z
 
 ### Added
+- Add a Dependabot action for updating GitHub actions (#452)
 - Use Coveralls for the code coverage (#425)
 
 ### Changed