add readme for contrib

Created by: jmitrevs

Description

This adds a readme to the contrib directory providing guidance on how to use it. Please check the guidance, and we can update it as suggested.

Type of change

• Documentation update

Tests

None: only a doc change

Checklist

• I have read the guidelines for contributing.
• I have made corresponding changes to the documentation.
• My changes generate no new warnings.
• I have installed and run pre-commit on the files I edited or added.