add readme for contrib

[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

• [x] Documentation update

Tests

None: only a doc change

Checklist

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