devel documentation how to do a release

[alban]

We could document how to do a release:

• check milestones
• build
• upload build artifacts on the github release
• run the cncf conformance tests: https://github.com/cncf/k8s-conformance/blob/master/instructions.md

Example of such documentation:
https://github.com/rkt/rkt/blob/master/Documentation/devel/release.md

Additionally, we can update the user documentation, how to use kube-spawn from a pre-compiled release.

/cc @robertgzr