From our meeting today, we are merging #335 with the following items left for a future PR:

• "Refers": add "field" or "to"?
• "Referrers": add "list"?
• Change defined words to lower case rather than upper case in the spec.
• Change "registry SHOULD accept a manifest with a refers" to "MUST". Include details of why (without saying GC).
• Change <reference> to <digest> and define <digest> in the Referrers API. Note that "tags may be supported in the future".
• Define an error code if the reference cannot be parsed in the Referrers API.
• Add "note" to "Multiple clients could attempt to update the tag simultaneously resulting in race conditions and data loss."