Tracking Status Hybrid

ACTION-145 ; ISSUE-107 ; ISSUE-120 ; ISSUE-47 ; ISSUE-61 ; ISSUE-124

Please find attached a proposal for a compromise between the well-known
URI and response-header approaches to server responses.

Edit highlights, compared to Roy's TSR text:
- added short non-normative introduction
- added compact version of the header
- fleshed out description of response values (& moved to own section).

Notes:
- We may need to modify the response values if we modify/clarify the
exceptions (permitted uses) list.
- It's in markdown for your reading convenience. I can HTMLify later.

I think that in-person small-group discussion is probably the most
effective way to hash out concerns with this.