Backport 688: Updating readme and webhook tempate to capture changes made for volumesnapshotclasses

[shawn-hurley]

What type of PR is this?
/kind documentation

What this PR does / why we need it:
Updating the readme based on the new kubeconfig flag for the webhook and webhook template such that the webhook will be used for volumesnapshotclasses as well

Cherry-pick #688: Action required: Added admission webhook to ensure that only one VolumeSnapshotClass can be default for each CSI driver. To benefit from this validation, please update your webhook configuration as shown in deploy/kubernetes/webhook-example/admission-configuration-template.```

[k8s-ci-robot]

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[humblec]

nit: space between stringon

[humblec]

This seems confusing to me. Default can be one. But is it conveying a message that, only one snapshot class allowed per driver? may be not, but sharing the thought anyway. :)

[shawn-hurley]

for a driver, there can only be one volume snapshot class set as the default.

considering this is a backport, should this be fixed in the master branch and then re-backported or just fixed here?

[xing-yang]

There are 2 commits in #688. Can you backport both?

[xing-yang]

/ok-to-test

[shawn-hurley]

@xing-yang sorry about that, updated and waiting on tests to pass

[xing-yang]

Can you also add the release notes that were in PR 688?

[xing-yang]

/lgtm
/approve

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: shawn-hurley, xing-yang

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