Update package guidelines to allow for custom licenses

[Eeems]

https://github.com/toltec-dev/toltec/pull/182#discussion_r559049513

[danshick]

https://spdx.github.io/spdx-spec/appendix-V-using-SPDX-short-identifiers-in-source-files/

As described at the bottom of this page, we should probably accept custom licenses using the LicenseRef identifier.

[Eeems]

I'd prefer to use the PKGBUILD method of custom:name https://wiki.archlinux.org/index.php/PKGBUILD#license

[danshick]

I like that as well. Maybe also require that non-standard licenses have the full text included in the package.

[matteodelabre]

Related: #255