Codebase README
Your codebase on a project will keep evolving, hence the code should be self-documenting via design patterns, unit tests etc. You should document the other stable stuff - the things you don’t want to say to each developer repeatedly. A great place to do this is right in the codebase, in a good, old README file, at the top level of your project folder. This is usually the first file people see on a project, so there’s a big incentive to make it useful. And there are tons of resources to guide you to do this well.
Make sure you include sections like project title and description, installation and usage, visuals and code review etc.
There are two tools that’ll help you generate these files with the complete markdown syntax. All you need to do is pick the sections you need in your file.