add section about "license of intent" for public domain works

[risacher]

In an email (may 2021) I proposed the following text as recommended language for "licensing" the work of gov't employees:

“Portions of this software are works of the United States Government and are not subject to copyright protection in the Unites States. In other jurisdictions and/or subsequent to establishment of copyright protection as a Joint Work, this software is licensed under the terms of the XYZ license.”

This attempts to find a sweet spot; it's true that the works of a government employee (civilian or military) is not protected by copyright in the United States, but such a work is generally protected outside the United States under the Berne Convention and later treaties. I am proposing that we acknowledge gov't works as public domain, while still expressing a "license of intent". In theory, a third party could take this public domain software, add copyrightable material and relicense the Joint Work under other terms than the intended license. This might be annoying but doesn't prevent the government from continuing to release and maintain the OSS project, under the chosen terms - it just creates a fork under another (possibly proprietary) license.