A file can be introduced and named as CONTRIBUTING.md; this can be in the root folder, a /docs folder (or /.github but using this folder wouldn't make sense since the GitHub repository is a mirror).
To modify this file, it would probably be easier to have it documented on MediaWiki.org so anyone can edit it, e.g Skin:Cosmos/For developers. We can link to a page on MediaWiki.org the same way that the CODE_OF_CONDUCT.md file does.
Things to include:
- How to properly file a task
- Bug report, feature request
- How to submit a patch
- Conventions to follow (we're following Wikimedia coding conventions)
- How to work with the existing developers (there's a Discord server here)
- How to setup the Cosmos skin repository locally
- Extensions to also have installed locally with Cosmos (see also T265539 for helping automate this on Gerrit)
On a bigger note (but more out of scope), there was an issue found trying to help onboard Omega from the move of GitHub to Gerrit, as there are many new concepts introduced per the migration. Many of the guides on MW.org are outdated or not linked very well to each other, so the discoverability of these guides are poor. One of the main things that is outdated (but should ideally be the most up to date page out of every single page) is the guide on how to create an extension, and how to create a skin.