Include all CC licenses

[pachadotdev]

Dear @jennybc

Following https://github.com/r-lib/usethis/pull/870, I included CC licenses within usethis. This will be useful, for example, for R4DS translation projects (i.e., R4DS itself is under CC BY-SA 4.0) and other non-code projects and data packages.

I will send a PR linked to this feature request.

Best, Pacha

[pachadotdev]

hi! here's the PR https://github.com/r-lib/usethis/pull/1855 also, I think it would be valuable to reformat all the MD files with the licenses, to render those like this one: https://github.com/matthewmccullough/Creative-Commons-4.0-Markdown/blob/master/licenses/by-nc-sa.markdown I can do that in a separate PR

[hadley]

We'd prefer to do this more tactically, adding CC licenses where there's a clear need. In general, we do want to steer our users towards specific licenses rather than offering a smorgasbord of every possible option.