Add Pod Identity documentation in README

[arnavgup1]

Is this a bug fix or adding new feature?
Feature

What is this PR about? / Why do we need it?
Adds documentation for the Pod Identity feature used for customers that want to authenticate with the pod identity agent that is available through EKS Add-ons.

PR for the pod identity change: #1254

[k8s-ci-robot]

Hi @arnavgup1. Thanks for your PR.

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[seanzatzdev-amazon]

/lgtm

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: arnavgup1, seanzatzdev-amazon

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[seanzatzdev-amazon]

/ok-to-test

[kkopachev]

I have a question about the language added by this PR.
It seems that adding AmazonEFSCSIDriverPolicy policy to the cluster's node group's IAM role is essentially the same as third method using instance profile policy:
If my node has AmazonEFSCSIDriverPolicy on it, then any pod on that node can use it, regardless of EKS Pod Identity presence.
Am I missing something here?