There are two types of "README"s in this project. All of them share the attributes of being named README.md, written in GitHub Flavored Markdown, and generally providing an overview of a topic.
The canonical README.md is located at the top of the repo and generally follows this readme standard.
There are also several other non-standard readmes found in various directories whose purpose is to provide context for the contents of the directory they are contained within.
The project readme generally follows a standard format. Some other format could be adopted, but it is useful to lean on one.