Release v3.6.1

Changes

• Fork long-abandoned SourceForge code to GitHub.
• Adjust license info in LICENSE and CREDITS so GitHub can find it.
• Convert README to README.md, add background info, and extract docs/HISTORY.md.
• Move most existing files and documentation underneath docs/sourceforge.
• Create Changelog based on the original NEWS.html from SourceForge.
• Create docs/REFERENCE.md based on the original gtml_ref.html file.
• Apply defined-array.patch and includeliteral.patch from the Debian package.
• Update gtml script to reflect the new GitHub location.