Change license to LGPL-3 and add some implementation notes.

Change license to LGPL-3 and add some implementation notes.

The purpose of the code is to be incorporated into other web projects
(whether those are free or not). AGPL prevents combination with other
HTML and javascript that is under a weaker (or proprietary) license.
Better would be a lesser AGPL, but there is not GNU standard for that.
So LGPL-3 meets most of my requirements. If somebody modifies the
actual client code and conveys it, then they must release the changes
under LGPL-3 also.

Add some implementation notes in docs/notes.