Extensive re-write of legal section

Thought I'd better explain exactly why CC0 is best. Attribution stacking is a real problem for certain re-uses.

Mietchen 2012: http://wir.okfn.org/2012/01/27/attribution-stacking-as-a-barrier-to-reuse/

Klimpel 2012: http://openglam.org/files/2013/01/iRights_CC-NC_Guide_English.pdf
Consequences, risks and side-effects of the license module "non-commercial use only - NC"