Docs: best practices for components project README

Problem

Our documentation of CI components does not describe how the README.md file should be written, neither points to real world example to follow as inspiration.

More and more users will create components repositories and the README file is important to ensure that documentation largely looks consistent across projects.

Proposal

Document README best practices and possibly link real world examples for users to follow. Ping @dnsmichi for feedback.

Ideas

We could also document how to breakdown documentation into multiple files (e.g. per component)

README.md
templates/
  component-1/
    template.yml
    docs.md
  component-2/
    template.yml
    docs.md