Multicluster DNS (kubernetes#2577)

* adding skeleton for PR

* move over everything except PTR records

* Adding in PTR records

* move <clustersetzone> being non configurable info to definition section, update links

* update links again, more specific

* add link to clusterid

* Clean up prose flow a bit, and add some rationale on SRV and PTR records in the readme

* in overview explicitly reference dummy service implementation as a reason to not need PTR records at multicluster DNS level

* Copyediting and formatting

* set initial schema version

* getting a little picky about sticky caps on clusterset

* fix toc

* also add toc for specification.md

* adding note on dual-stack

* Change examples to use more readable clusterIDs and add in to headless SRV records for clarity

* Be more clear about what hostname means in this spec file directly

* allow flexibility on headless service DNS reporting a subset of pod IPs

* clarify ClusterIPs are not necessarily accessible clusterset wide

* fix zone->clustersetzone

* reserve disallowed DNS forms for future use

* for disallowed DNS records, they MUST NOT not just should not

* use better prose for how ClusterSetIP DNS clusterips work

* clustersetzone->clusterset-zone

* S A V A G E removed PTR recordsgit status k bye

* fix toc

* fix toc again

* add subset language to actual specification.md for headless service DNS

* wording nits and follow RFC 1123 my own self

* some formatting and clarify prose on subset used for headless A/AAAA records in dual stack case

* rewrap specification.md

* rewrap MCS API doc, praying the toc is healthy

* toc did not like getting rewrapped yall

Author: lauralorenz