The user guide has a short section on writing custom rules [1], and the API reference also covers the attributes of a LintRule [2]. I plan to document this in more detail at some point in the future.

1: https://fixit.readthedocs.io/en/latest/guide.html#custom-rul...

2: https://fixit.readthedocs.io/en/latest/api.html#lint-rules