Document the mapping

[ForNeVeR]

We need to figure out how exactly the maps were created.

There's a document on mapping here, but it may be outdated.

Also, since #78, the mappers are also required to run glbsp on their maps.

Let's see what of this may be simplified (i.e. a pre-packaged Doom Builder installation or whatever), and also let's see into glbsp usage and whether it's possible to repack the relevant parts with Cesium, for the game to pick up maps from the editor without manual run of glbsp.