The stackdriver documentation could inform better the users of workload identity

Currently the readme says

> If you are running on **Google Kubernetes Engine**, you must explicitly add https://www.googleapis.com/auth/cloud-platform to the API access scopes

And this works when using kubernetes in most cases.

However when using workload identity ON, this is not enough

The service account linked to workload identity must have some roles added to it:

```ruby
"roles/logging.logWriter",
"roles/cloudtrace.agent",
"roles/errorreporting.writer",
"roles/stackdriver.resourceMetadata.writer",
"roles/monitoring.metricWriter"
```

so maybe this can be added in the read me too.

This is important because workload identity is the recommended way to use on gke and also it is on the way to become the default behaviour in the future.