Document usage of dependabot for keeping GH Action up to date.

[tmcgilchrist]

It would be useful to point people to a simplified dependabot configuration to keep the setup-ocaml action up to date. Bonus points for the project it keeps everyone up to date with minimal fuss.

For example

version: 2
updates:
  - package-ecosystem: "github-actions"
    directory: "/"
    schedule:
      interval: "monthly" 

in a file under .github/dependabot.yml in your project will check monthly for newer versions of GH actions. Creating a PR updating the configuration if you do.

[smorimoto]

This sounds good, but Dependabot often prioritises full semver style tags, which I worry could be noise.

[smorimoto]

Addressed in the latest master.