Updated README to include subordinate template for clientAuth

[weiwarren]

Updated document to be consistent with AWS PCA templates. These are important information for users when using CA issuers in k8s in order to generate certificate for mTLS between pods.
https://docs.aws.amazon.com/privateca/latest/userguide/UsingTemplates.html#template-varieties

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[weiwarren]

/assign @paramsethi

[bmsiegel]

Hi @weiwarren, this README change is actually incorrect. We map the certmanager usageType to a template in a 1:1 fashion (https://github.com/cert-manager/aws-privateca-issuer/blob/main/pkg/aws/pca.go#L204). We have a feature request to support more path lengths here (#98). Please feel free to reopen or +1 this issue. If you've implemented something for it we're certainly willing to take a look as well!

[bmsiegel]

Closing this PR for now due to no response