Add a `readme.txt` file for the theme

[dustin-jw]

The theme check plugin can be used to validate themes against a test suite to report any issues with the theme.

We get these errors that would be addressed by adding a valid readme.txt file, like the one for the twentytwentythree theme.

WARNING: Could not find a copyright notice for the theme. A copyright notice is needed if your theme is licenced as GPL.
REQUIRED: Could not find the file readme.txt in the theme.