Update uid/gid Readme

[Ashley-wenyizha]

Is this a bug fix or adding new feature?

Doc update - Document existing access point user identity enforcement behaviors in README.

#748

^^ As Daniel is blocked by the EasyCLA issue, re-submitting this PR through my account.

[k8s-ci-robot]

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[wongma7]

/lgtm

/test pull-aws-efs-csi-driver-verify

[Ashley-wenyizha]

failed test indicates to run hack/update-gofmt, however, after running hack/update-gofmt & ack/verify-gofmt this makes not formatting changes

wenyizha@3c22fb3758da aws-efs-csi-driver % hack/verify-gofmt 
Verifying gofmt
No issue found

wenyizha@3c22fb3758da aws-efs-csi-driver % hack/update-gofmt 
wenyizha@3c22fb3758da aws-efs-csi-driver % git status        
On branch DocUpdate
Your branch is up to date with 'origin/DocUpdate'.

nothing to commit, working tree clean

[wongma7]

/lgtm