enable dependabot for GitHub actions (#446)

This allows to get updates for GitHub actions automatically. I have used
this for my own packages, the [Trixi.jl
framework](https://github.com/trixi-framework), and the [SciML
organization](https://github.com/SciML). 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

Co-authored-by: Julius Krumbiegel <22495855+jkrumbiegel@users.noreply.github.com>

Author: ranocha

.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: "weekly"