SPDX 3.0 Support

[goneall]

This issue is to provide feedback on the SPDX 3.0 design and implementation prior to the pull request being created.

The v3-prototype branch is being used for development.

There is a README for the upgrade - feedback on this document would be much appreciated. I'm particularly interested if there are things we could implement to ease the migration to 3.0 as there will be breaking changes.

[sschuberth]

Any ETA for 3.0 support now that the spec is final?

[goneall]

@sschuberth - I don't have a specific date, but there is a prototype under development. Now that the heavy lifting is done on the spec, I'll shift my focus to updating the library. Hope to provide an update on progress / timing within the next week or two.