Add OWNERS file

[paulfantom]

There should be an OWNERS file (like this one) to allow prow to manage who is allowed to review and/or approve PRs.

[wking]

It would be nice if this came with docs to make it more discoverable and explain how the file fits into project governance. For example, the k8s template has a brief reference from CONTRIBUTING.md and (stale) links from the OWNERS file to upstream docs explaining how it works (kubernetes/kubernetes-template-project#15). CRI-O has a similar CONTRIBUTING.md line, which contains links to both OWNERS and (different) upstream docs.

[stevekuznetsov]

The most up-to-date documentation is here but we understand the documentation is not perfect today and we are working on making it better.