Document the need of audience in token when connecting to secure metric endpoint

[vprashar2929]

Since we are using the audience as a required thing when passing the token #554 (comment)

We should document this in our release notes upstream and downstream so that it is clear to the users that they have to embed the audience in their token when requesting

[KaiyiLiu1234]

Is there anywhere else we should document thoroughly how to use kube rbac proxy?

[vprashar2929]

In README upstream and maybe in our downstream docs