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: https://github.com/kubernetes-sigs/aws-efs-csi-driver/pull/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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Needs approval from an approver in each of these files: - ~~[OWNERS](https://github.com/kubernetes-sigs/aws-efs-csi-driver/blob/master/OWNERS)~~ [seanzatzdev-amazon] Approvers can indicate their approval by writing `/approve` in a comment Approvers can cancel approval by writing `/approve cancel` in a comment

[seanzatzdev-amazon]

/ok-to-test