Add the SPDX 2.2 specification we follow, as well as the acceptable licenses in
the FreeBSD tree to be referneced indirectly. Write up this policy and include
it in the README.txt file.
Please note: BSD-2-Clause-FreeBSD and BSD-2-Clause-NetBSD have been omitted
from this list as they have been deprecated by SPDX. It's unclaer what the
proper thing to do here is, so err on the side of caution by requiring the
exact licenses to be included in these files for now.
Also note: Do not go removing license text from files in the FreeBSD
tree just yet.