Add copyleft explanation.

This has been hanging out uncommitted for who knows how long...
Finish mapping decisions for copy-left licenses.
Add attribution menu.
Add license table to README.

I don't have internet at the moment, nor do I have most of these licenses
on-hand.  I recall less of their details than I thought I would, and I'd rather
not start structuring things based on inaccurate information... so this is a
pause for now.