Github will display a link to it when a user wants to create an issue or
pull request.
Also make some minor adjustments to DOCS/contribute.md, which is
developer oriented, and for which I see no reason to merge it with
the new file.
As suggested in #2033.
Additionally, change "LGPLv2+" to "LGPLv2.1+". Since this was always
used with the "and later" phrase, this doesn't require any further
discussion.
Also clarify that the "+" means "or later". This should be clear to
anyone with a brain, but you never know with copyright.
