Cut distribution-spec release with reference types

[lachie83]

Now that the reference types working group proposal has been merged into main, I would like to start the discussion on getting image-spec and corresponding distribution-spec releases cut. The reference types work was merged via the following PRs:

• Reference Types Working Group remaining tasks image-spec#940
• Working Group Proposal for Reference Types #335

There is also some cleanup work covered in the following issue - #337

In order to ease the decision of when to cut a release, I thought creating a checklist of items that must happen prior to release (release gating criteria if you will) might be a good way to make progress towards that goal. I have come up with the following list and am happy to add more items that might have been missed.

• Closure of outstanding tasks
• Conformance tests
• Sufficient Documentation
• OPTIONAL Implementation endorsements (potential list of registry providers that have agreed to implement the changes)
• Versioning agreement (if/how these changes to the image-spec constitute an API version change)

[sudo-bmitch]

I'm closing this one out since we finally had a 1.1.0 release earlier this year. Thanks for all your help getting us there @lachie83!