Add staging directory for pod-security-admission

[tallclair]

What type of PR is this?

/kind feature

What this PR does / why we need it:

Add the staging directory for the pod-security-admission repo. See kubernetes/org#2713 for more detail.

Which issue(s) this PR fixes:

kubernetes/org#2713

Special notes for your reviewer:

I used 66284b2#diff-1c81d24def247f1dfee702078c4ea1f30bea58dc3bfacb172644933c99480510 as a template to guide this.

Does this PR introduce a user-facing change?

NONE

Additional documentation e.g., KEPs (Kubernetes Enhancement Proposals), usage docs, etc.:

- [KEP]: https://github.com/kubernetes/enhancements/tree/master/keps/sig-auth/2579-psp-replacement

/sig auth
/sig architecture

/assign @liggitt @dims

[k8s-ci-robot]

@tallclair: This issue is currently awaiting triage.

If a SIG or subproject determines this is a relevant issue, they will accept it by applying the triage/accepted label and provide further guidance.

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[liggitt]

/lgtm
/approve

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: liggitt, tallclair

The full list of commands accepted by this bot can be found here.

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• OWNERS [liggitt]

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[tallclair]

/hold

[tallclair]

Name updated.

[liggitt]

/lgtm

[liggitt]

/hold cancel