Create a dependabot.yml for Github actions (p4lang#4405)

* Create a dependabot.yml for Github actions

* Remove some unnecessary comments

.github/dependabot.yml

@@ -0,0 +1,10 @@
+# Please see the documentation for all configuration options:
+# https://docs.github.com/code-security/dependabot/dependabot-version-updates/configuration-options-for-the-dependabot.yml-file
+
+version: 2
+updates:
+  - package-ecosystem: "github-actions" # See documentation for possible values
+    directory: "/" # Location of package manifests
+    schedule:
+      interval: "weekly"
+    open-pull-requests-limit: 5