Documentation: Add a guide on how to create custom maps

[htho]

[Custom] maps can be created by yourself with SAS.Planet application from mostly any online source or converted with MapTiler and other applications from other map formats.

I used an app where I could download many different maps including satellite pictures and add/mix them with other maps. I am pretty sure it was Trekarta, but this list of additional maps is missing now. (Maybe due to copyright stuff?).
When I look at the screenshot from the readme, I see what I want, but I am not Map savvy enough to create my own custom maps. It would be nice to have a guide that helps me do that and also lists map/data resources I can use tm create my custom maps.