Minor Editorial tweaks: Update Status to "Approved Deliverable" and add "Known Implementations"

[bumblefudge]

1.) The status in the spec still reads "DIF Working Group Approved Draft" but the SC approved it. The DIF Charter official title for this status is "Approved Deliverable". I'm happy to dive into the ReSpec and open the PR myself, if the chairs prefer.

2.) Likewise, there was some discussion on Slack about adding a "Known Implementations" section with links to live .wellknowns. For now, I added these to the readme.md here on github-- should they be moved into the spec, or should the spec link directly to the readme section? Again, happy to do myself if it's non-controversial, but consensus from chairs preferred :D

@csuwildcat @tplooker @OR13