ClassicPress Core does not read the readme file, thus, it is technically not required.
The ClassicPress Theme Directory, however, will use the description provided in the README.md or readme.txt over the one provided in theme file style.css. Therefore it’s recommended to add a readme file in the main directory of your theme. This also makes it possible to provide a more extensive description of your theme, as you can see below. ClassicPress prefers a README.md over a readme.txt file.
The markup of the README.md file differs from the one WordPress uses.
The (redacted) example below is the readme from The ClassicPress Theme.
# The ClassicPress Theme The ClassicPress Theme is the default ClassicPress theme and is based on the theme used on the ClassicPress website at https://www.classicpress.net. The ClassicPress Theme is designed to be used in three ways: - as is, with customizations made using the Additional CSS box in the customizer and/or via plugins; - as a starter theme, where you customize any of the code as you wish, provided that you also rename the theme and the directory in which it resides so that it does not get accidentally overwritten; or - as a parent theme, which you customize by creating a child theme. ... ## Special Features The ClassicPress Theme comes with two accessible menus. The top one is for the main navigation. It automatically becomes a mobile menu with a search box on narrow screens. There is also a menu for use in the footer, if desired. ...
This will result into this:
