Add deprecation notice to README for archival

[aburdenthehand]

This repo is being archived due to inactivity as part of kubevirt/project-infra#3686
/cc @dhiller

[kubevirt-bot]

Adding the "do-not-merge/release-note-label-needed" label because no release-note block was detected, please follow our release note process to remove it.

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[kubevirt-bot]

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