chore(docs): update Workload Identity README example for existing GSA (#1062)

Addresses #1059 -- without the `depends_on` block, the WI module will
try to read a data source for a Google service account that has not yet
been created. This is alluded to in the Workload Identity examples but
not in the Workload Identity README. This commit updates the README to
include the `depends_on` block.

Co-authored-by: Morgante Pell <[email protected]>

Author: tomasgareau

examples/workload_identity/main.tf

@@ -95,5 +95,6 @@ module "workload_identity_existing_gsa" {
   name                = google_service_account.custom.account_id
   use_existing_gcp_sa = true
   # wait till custom GSA is created to force module data source read during apply
+  # https://github.com/terraform-google-modules/terraform-google-kubernetes-engine/issues/1059
   depends_on = [google_service_account.custom]
 }

modules/workload-identity/README.md

@@ -59,6 +59,10 @@ module "my-app-workload-identity" {
   use_existing_gcp_sa = true
   name                = google_service_account.preexisting.account_id
   project_id          = var.project_id
+
+  # wait for the custom GSA to be created to force module data source read during apply
+  # https://github.com/terraform-google-modules/terraform-google-kubernetes-engine/issues/1059
+  depends_on = [google_service_account.preexisting]
 }
 ```