overlays/default: Add README and equery-licenses

overlays/default: Add README and equery-licenses

The packages distributed in the release ISOs are distributed under a
range of licenses, but in the ISO we distribute them without their
source.  Since many licenses (e.g. the GPLv2, section 3) require you
to also distribute or provide access to the corresponding source code,
it's good to be clear about the licensing of the bundled binaries.

The large list of installed packages and licenses can also serve as a
teaching tool for explaining how open source development works, and
emphasizing why using a standard license is a good idea ;).