Describe how to read a Pub/Sub topic from another project

The GOOGLE_CLOUD_PROJECT environment variable needs to be set to the project where the Pub/Sub topic resides.

This addresses #1811

Author: jm-franc

cost-optimization/gke-keda/cloud-pubsub/deployment/keda-pubsub-with-workload-identity.yaml

@@ -41,6 +41,8 @@ spec:
     spec:
       serviceAccountName: keda-pubsub-sa
       containers:
+      # Note: Set the GOOGLE_CLOUD_PROJECT environment variable if the Pub/Sub topic
+      # is not in the same GCP project as this Kubernetes cluster.
       - name: subscriber
         image: us-docker.pkg.dev/google-samples/containers/gke/keda-pubsub-sample:v1
 # [END gke_deployment_keda_pubsub_with_workflow_identity_deployment_pubsub]