Add porters-handbook for USE_GNOME.

This contains two chapters. The first chapter explains USE_GNOME and
the macro's and the second chapter which lists all the possible
components with a short description.

Loosely based on the old porting guide (http://freebsd.org/gnome) and
the Qt chapter. The old porting guide will stick around for now until
I get around to updating the gnome website.

Reviewed by: mat@, wblock@
Approved by: mat@
Differential Revision: https://reviews.freebsd.org/D3577