Write upgrade guide for 1.1 -> 1.2

[elichad]

Write some guidance on how to transition from 1.1 to 1.2 spec, including clear statements of which requirements are changed/new/removed, and including brief descriptions of motivation/implications of changes where this might be useful.

This will be useful for RO-Crate users who do not have a deep familiarity with the spec, and will also help to guide updates to tools like ro-crate-py (see ResearchObject/ro-crate-py#211) and rocrate-validator.

Use the existing changelog as a starting reference point.

Assigning to myself as I plan to work on this in the next couple of weeks. Does not block the release of 1.2.