Documentation bugs in custom-metrics-stackdriver-adapter README

I'm just starting to follow the [Custom Metrics - Stackdriver Adapter](https://github.com/GoogleCloudPlatform/k8s-stackdriver/tree/master/custom-metrics-stackdriver-adapter#custom-metrics---stackdriver-adapter) installation instructions (got here from [the GKE doc on 'Optimize Pod autoscaling based on metrics'](https://cloud.google.com/kubernetes-engine/docs/tutorials/autoscaling-metrics#pubsub)), and already noticing a few odd issues with the docs:

* Recommendation to add scopes is likely irrelevant given the content of [this doc](https://cloud.google.com/kubernetes-engine/docs/how-to/access-scopes) - we probably want to update this section, link to this doc about access scopes and perhaps replace the oauth instructions with a mention of configuring IAM for the service account associated with the cluster (we can link to the latter section fo the same doc).

For context, the document begins with:
> Access scopes are the legacy method of granting permissions for your nodes, and for workloads running on your nodes if the workloads use [application default credentials (ADC)](https://cloud.google.com/docs/authentication/application-default-credentials). Access scopes define the OAuth scopes used in API calls from client libraries or the gcloud CLI.

* The section on 'Workload identity' is missing words, here's an excerpt:

> If you use Workload Identity in your cluster, additional steps are necessary. In the commands below, use your Project ID as and Google Service Account as .

Given the warning in the root of the repo, are contributions welcome? Also, is it even recommended to follow this guide, or is there a way to install the adapter automatically in GKE?