Missing M3 themeoverlay description in export

[patrickfrei]

Describe the bug When exporting the M3 themes on https://material-foundation.github.io/material-theme-builder/ for "Android Views (XML)", the according ZIP file contains a README file.

In this README file, there is no clear description on how to implement the theme_overlays.xml files. E.g. folder, exact handling in the code, etc. I also couldn't find a description about the using of these files on the M3 website.

Could you please add it? Many thanks!