Update PR clause to include attribution.

Signed-off-by: Engel Nyst <engel.nyst@gmail.com>
Add some sort of compatibility with CC-by-SA, using the language of Diaspora
CLA.
(https://github.com/diaspora/diaspora/wiki/New-CLA--12-13-10)
The text seems weaker than the OSI compat paragraph (shouldn't it be
sure to be stricter in the limitation of the covered works?), but it
will do for starters (and so that I remember it :P).

Signed-off-by: Engel Nyst <engel.nyst@gmail.com>