multicmake info added

[davidsanfal]

@memsharded I don't know if this is enough info or if it's clear, but I couldn't think of a way to explain it in a few lines without ending up giving a complete example of how to use it.

[czoido]

I think this section is a bit misleading right now. The example mixes the source folders (where the CMakeLists.txt live) with the output subfolders created.

From what I understand:

• build_script_folder → points to the location of the CMakeLists.txt inside the source tree (e.g., cmake_one/, cmake_two/).
• subfolder → defines an extra directory name inside both the build_folder to isolate the outputs (e.g., one/, two/).

So the text should make it explicit that the CMakeLists.txt are in the cmake_one/ and cmake_two/ source dirs, while the build artifacts go under build/one/, build/two/.

I would try to make this clear in the example.

Also, I think this needs to be marked as experimental.