Update spec; archive repo?

[dontcallmedom]

With this proposal no longer being worked on, it would be good that, in addition to the README, the spec itself be updated to reflect that status.

It may also be useful to archive the repo if no work is expected to happen on it.

[yoavweiss]

@jkarlin - Are you cool with archiving this?

[marcoscaceres]

Updated the document to point to https://github.com/patcg-individual-drafts/topics

Archiving.