Include reference to currently supported constructs in our validation errors

[wking]

Currently, crates.io wraps our errors (e.g. here), which can result in confused users unless the references are specific. And if crates.io decided to include specific references (like Cargo grew in rust-lang/cargo#4898) we wouldn't be particularly DRY. I think this repository should clearly document what is supported and (optionally?) include a link to those docs in our error messages. We'd need to pin the link to a particular license-exprs release, but a message like:

unknown license or other term: NotInSPDX.  See https://github.com/rust-lang-nursery/license-exprs/blob/v1.4.0/README.md

would seem to cover rust-lang/cargo#3540 in a maintainable fashion. One we get clearer docs, we can update the Cargo docs to point at our docs as well, and then we'd be DRY. Thoughts?