Plugin README Requirements

ClassicPress Core does not read the readme file, thus, it is technically not required. Only the WordPress Repository parses, uses and sends this information to the “more details” pop up in your install when installing the plugin.
However, the developer probably has to communicate some information to the user, which the README is ideal for.
If you include a README file, you can use a readme.txt or readme.md

Since ClassicPress has no specific requirements or usage for Plugin Readme Headers, you can follow the WordPress requirements if you plan to add one.