Task: Refine the doc for PR #27

[erikmd]

• Related: PR feat: Make opam_file optional #27

Posted by @Zimmi48 in #26 (comment):

Since opam_file is now optional, this means that the most simple workflow configuration corresponds to the one I wrote in my comment above (except that we can compress the on: part even more). I think that showing such a simple (complete) configuration somewhere near the top of the README would look great (to show how simple it is to use Docker-Coq-Action for simple use cases).