Create dependabot.yml (#4)

This allows to get updates for GitHub actions automatically - it is basically an equivalent of CompatHelper. I have used this for my own packages, the Trixi.jl framework, and the SciML organization. After merging this, you could also enable other Dependabot actions in 'Settings -> Code security and analysis -> Dependabot alerts' and '... -> Dependabot security updates'.

See SciML/MuladdMacro.jl#37

.github/dependabot.yml

@@ -0,0 +1,7 @@
+# https://docs.github.com/github/administering-a-repository/configuration-options-for-dependency-updates
+version: 2
+updates:
+  - package-ecosystem: "github-actions"
+    directory: "/" # Location of package manifests
+    schedule:
+      interval: "monthly"