Understand ... searching ... best should be to place a link (to file, URL) directly into the theme. But this makes sense only in case, I add this also into the official MapsForge4 library and it may take some time ...
So in the next version, "temporary" solution: currently PDF (any other format may be useful as well?) file with the same name and location as a theme XML file. In the list of themes, such a theme will have a small badge and long-click offer display of legend.