Add guidelines to contribute code to Mediawiki Codesniffer. There are a few subtleties like making _pass and _fail tests for Sniffs, making sure Codesniffer passes phpcs script ( otherwise main build breaks ), etc. This can be added as a subheading in the readme itself.
I think this sort of documentation should be at https://www.mediawiki.org/wiki/Manual:PHP_Codesniffer (and not be WMF-specific). At the moment there's https://www.mediawiki.org/wiki/Continuous_integration/PHP_CodeSniffer but that's meant to be just about how to add phpcs to CI (a lot of info was removed from that page a couple of years ago).