Add offline note for kube-* images

[oomichi]

What type of PR is this?

/kind cleanup

What this PR does / why we need it:

We had a question1 why kube-* images need to be handled separately when getting container image list for offline deployment.
This adds the note to explain it for long-term maintenance.

Does this PR introduce a user-facing change?:

NONE

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: oomichi

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[oomichi]

/cc @tmurakam @muzi502

[k8s-ci-robot]

@oomichi: GitHub didn't allow me to request PR reviews from the following users: tmurakam, muzi502.

Note that only kubernetes-sigs members and repo collaborators can review this PR, and authors cannot review their own PRs.

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/cc @tmurakam @muzi502

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[oomichi]

To see comments from the script authors.

/hold

[cristicalin]

@oomichi considering we may not get an answer on this we might as well merge it.

[oomichi]

@oomichi considering we may not get an answer on this we might as well merge it.

OK, this comment might be fine.
Let me drop hold.

/hold cancel

[tmurakam]

Sorry for late reply, looks good to me.

[cristicalin]

@oomichi 👍
/lgtm