spdx: document usage

Document that we try to follow SPDX 2.2 for the SPDX-License-Idneitfier: tags
that are in the source. Note that they are informative only. Give pointers to
the standards and a status of the files marked in-tree.

Suggestions by: Pau Amma
Differential Revision: https://reviews.freebsd.org/D29330