Document cross-region ECR support

[therc]

Fetching images from a registry hosted in a different region has been supported since 1.3 with
kubernetes/kubernetes#24369

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[therc]

I signed it!

[chenopis]

@therc can you make a slight change and add an 'is' in the sentence, i.e. "If your nodes are in region A and your registry is in a different region B..." Thx

[chenopis]

@therc Nevermind, I can add it in a separate change.