Add repo SECURITY.md

[joelsmith]

See kubernetes/committee-security-response#105

[RainbowMango]

/lgtm
/assign @madhusudancs @pmorie

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[nikhita]

Closing since the security policy is now defined org-wide - kubernetes/org#2081

For this repo, it can be seen at https://github.com/kubernetes/cluster-registry/security/policy