Update README on listing multi architecture images

[CatherineF-dev]

What this PR does / why we need it:
After migrating from k8s.gcr.io to registry.k8s.io, it's hard to find multi arch images.

How does this change affect the cardinality of KSM: (increases, decreases or does not change cardinality)

Which issue(s) this PR fixes (optional, in fixes #<issue number>(, fixes #<issue_number>, ...) format, will close the issue(s) when PR gets merged):
Fixes #2069

[k8s-ci-robot]

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[CatherineF-dev]

cc @mrueg

[mrueg]

/lgtm

thanks!

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: CatherineF-dev, mrueg

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