Documentation is important for themes as it provides a way for users to understand what a theme does and does not support. Likewise, documenting the code of your theme will make it easier for other theme developers to customize your theme, likely with a child theme.

Here’s a list of requirements and recommendations for your theme’s documentation.

• Themes are required to provide end-user documentation of any design limitations or extraordinary installation/setup instructions.
• Themes are required to include a readme.txt file, using the plugin directory’s readme.txt markdown format. New themes need to follow this rule as of October 25th, 2018. Old themes have a 6 months grace time from this date. Since WordPress 5.8 theme readme files are not parsed for requirements. This means that headers Requires PHP and Requires at least are going to be parsed from theme’s style.css file.