Document sublicensing non-CC-BY materials in the paper, and licensing generally

Just opening the issue from slack discussion, raised by @RichardLitt and also @mstimberg > What is the JOSS policy around licensing for images used in a publication? Are authors allowed to include images which are effectively CC-BY-NC? The Stanford Bunny is included in one of the papers I'm editing, and the website says you can't use this in pieces for commercialization. https://graphics.stanford.edu/data/3Dscanrep/3Dscanrep.html#bunny I could find nothing in the JOSS docs about this. More generally we need documentation about - how JOSS papers are licensed (there isn't a mention on the submission page about the license that the paper will have, or in the policies). this is present on the main website, but should also be in the docs - https://joss.theoj.org/about#content_license - what is licensed as CC (is the `paper.md` supposed to also be CC-BY? what about images or other materials?) and how to handle sublicensing within a repository - whether it's possible for authors to select a CC license (this would be great to have, currently i don't think we support this, but it would be pretty easy to) - how to handle license conflicts from included materials (per the original question) - ... what else? Related issues: - #343 - #1474