Added COPYRIGHTS file to track copyright status.
Added COPYRIGHTS file to track copyright status.

This'll be handy for the Debian packaging.

vignanl/Plinth,jvalleroy/plinth-debian,vignanl/Plinth,kkampardi/Plinth,vignanl/Plinth,kkampardi/Plinth,jvalleroy/plinth-debian,vignanl/Plinth,harry-7/Plinth,vignanl/Plinth,jvalleroy/plinth-debian,kkampardi/Plinth,harry-7/Plinth,freedomboxtwh/Plinth,harry-7/Plinth,jvalleroy/plinth-debian,kkampardi/Plinth,freedomboxtwh/Plinth,harry-7/Plinth,harry-7/Plinth,jvalleroy/plinth-debian,freedomboxtwh/Plinth,freedomboxtwh/Plinth,freedomboxtwh/Plinth,kkampardi/Plinth