Add "Implementation" notes to RFCs? #604

I suggest besides Reviews, Responses, Comments and Versions, we also add an "Implementations" category in the RFCs.

The "Implementation" report would provide an snapshot of the current state of one particular implementation of an RFC.

This would help moving RFCs to accepted (requires 2 initial implementations) and, after spec changes, to adopted (2 complete implementations).

Pros:

• Public documentation of implementations in context of particular RFCs
• Helps tracking and acknowledgment of implementers
• Supports automatic RFC status detection

Cons:

• One more thing to write and manage
• Implementations move faster and document gets outdated

Motivated by

• add RFC-4 review 3b #603 (ping @kevinyamauchi)
• Restructure RFC documents to use a YAML front matter for Status and author/reviewer/commenter metadata #552

I'll ping @ome/image-coop as implementation of RFCs is a core business of the Coop