Add requirement to annotate K8s service account when using workload identity

[bradjones1]

TL;DR

The README instructions should also contain a step to annotate the Kubernetes service account, per the docs.

Expected behavior

Workload identity is set up sufficient to use the feature.

Observed behavior

The docs for this project only contain the step to bind the service accounts, but not to annotate the Kubernetes object.

[dargudear-google]

There is update in the doc, can you please check once?