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

documentation/content/en/articles/committers-guide/_index.adoc

@@ -3427,6 +3427,20 @@ Once the {core-email} is satisfied that all the necessary details have been gath
 
 The license archive should contain only details of license grants; this is not the place for any discussions around licensing or other subjects. Access to data within the license archive will be available on request to the {core-email}.
 
+[[spdx.tags]]
+== SPDX Tags in the tree
+
+The project uses https://spdx.dev[SPDX] tags in our source base.
+At present, these tags are indented to help automated tools reconstruct license requirements mechanically.
+All _SPDX-License-Identifier_ tags in the tree should be considered to be informative.
+All files in the FreeBSD source tree with these tags also have a copy of the license which governs use of that file.
+In the event of a discrepency, the verbatim license is controlling.
+The project tries to follow the https://spdx.github.io/spdx-spec/[SPDX Specification, Version 2.2].
+How to mark source files and valid algebraic expressions are found in https://spdx.github.io/spdx-spec/appendix-IV-SPDX-license-expressions/[Appendix IV] and https://spdx.github.io/spdx-spec/appendix-V-using-SPDX-short-identifiers-in-source-files/[Appendix V].
+The project draws identifiers from SPDX's list of valid https://spdx.org/licenses/[short license identifiers].
+The project uses only the _SPDX-License-Identifier_ tag.
+
+As of March 2021, approximately 25,000 out of 90,000 files in the tree have been marked.
 [[developer.relations]]
 == Developer Relations