Document Kubernetes Release Process

This documentation covers the proposed release process that will improve
support for versioning builds from tarball.

There's also a diagram to explain how versioning works in face of other
commits being merged in parallel and the quirks of intermediate versions
close to the release window.

Addresses Issue #1226.

Signed-off-by: Filipe Brandenburger <filbranden@google.com>
Merge pull request #1237 from filbranden/release_docs

Document Kubernetes Release Process