By default this library is needed as the option overlay-xcompile
defaults to ON. Therefore we should list g++-multilib as a dependency
directly and only mention how to get rid of this dependency, not the
other way around.
This will probably avoid some confusion on the reader's side.
The existing `docs` directory was restructured a bit and the build
documentation previously located at
https://github.com/mumble-voip/mumble-releng-vcpkg was integrated into
the main repository (after having received a good polish).
In addition to that the needed scripts and port-files from the linked
repository have now also been included in the main repo.
