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)
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 Many of the guides on MW.org are outdated or not linked very well to each other, so the discoverability of these guides are poor. One of the main things that is outdated (but should ideally be the most up to date page out of every single page) is the guide on how to create an extension, and how to create a skin.