Automatic merge from submit-queue.
Add the cluster registry API design to the community repo
/assign @quinton-hoole @csbell
/cc @pmorie @font @ericchiang
This is a mechanical conversion of the [Cluster Registry API doc](https://docs.google.com/document/d/1Oi9EO3Jwtp69obakl-9YpLkP764GZzsz95XJlX1a960/edit), with a few tweaks to better suit the format and to note the provenance of this document.
Quinton and Christian, I'd like both of you to sign-off on this before I commit. Anyone else is free to chime in if they have concerns, but I believe that we achieved consensus on this doc, and I'd like to formalize that.
Christoph Blecker
Wiiliam Chang
Jonathan MacMillan
Kubernetes Submit Queue
Xingcai Zhang
guineveresaenger