Quoting Miroslav Rovis (miro.rovis@???):

> However, while you are right in that the CC-BY-NC (which license you
> noticed that I have used up until now; will be changing to CC-BY
> A.S.A.P.) [in that the CC-BY-NC] is (for short:) incompatible with free
> licenses (GPL and the broad family), CC-BY is compatible...

CC-BY is indeed a free-software / open source licence (designed for
non-software cultural works) -- one of several such from Creative
Commons. CC BY-SA might also be of interest, being reciprocal
(copyleft), and is the one I use most often.

You may find CC's summary page usef;
https://creativecommons.org/licenses/