test: Update images README and add OWNERS

[smarterclayton]

Document the review criteria as part of accepting rebase.

[bparees]

Can we get some documentation that explains what this process actually does?

Things i'm confused about:

1. if i add a new test that uses a new image (assume i have a good reason), does that image get listed in images.go's init function as an exception? I wouldn't think so, but the current docs seem to tell you to do that. I would have assumed that the update tooling would just "find" my image reference in my test and add it to the zz_generated.txt

2. does something being listed in zz_generated.txt result in it being mirrored? What is doing that mirroring and when?

3. What actually happens at runtime? My impression is that the idea behind this is that all the image strings in test code get substituted w/ the mirror (quay.io or whatever is specified on the command line) location prior to test execution? Is that accurate?

I also put some questions in https://github.com/openshift/origin/pull/24887/files#r538841935 that could use clarification.

[soltysh]

/lgtm

[soltysh]

/lgtm

[openshift-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: smarterclayton, soltysh

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[bparees]

Suggested change

	* Reference your image inside your tests tests using `github.com/openshift/origin/test/extended/util/image.LocationFor("my.source/image/location:versioned_tag")`
	* Reference your image inside your tests using `github.com/openshift/origin/test/extended/util/image.LocationFor("my.source/image/location:versioned_tag")`

[bparees]

To the best of my understanding CI builds don't push to quay.io nor to the openshift namespace, so i'm confused by this.

CI builds push to registry.svc.ci.openshift.org and generally in the ocp namespace (though certainly they can push to other namespaces)

[openshift-merge-robot]

@smarterclayton: The following test failed, say /retest to rerun all failed tests:

Test name	Commit	Details	Rerun command
ci/prow/e2e-agnostic-cmd	20dc4ec	link	/test e2e-agnostic-cmd

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