Add migration instructions to doc

Author: morgante

docs/upgrading_to_v5.0.md

@@ -5,6 +5,37 @@ release.
 
 ## Migration Instructions
 
+### Resource simplification
+The `google_container_cluster` and `google_container_node_pool` resources previously were different between regional and zonal clusters. They have now been collapsed into a single resource using the `location` variable.
+
+If you are using regional clusters, no migration is needed. If you are using zonal clusters, a state migration is needed. You can use a [script](../helpers/migrate.py) we provided to determine the required state changes:
+
+1. Download the script
+
+    ```
+    curl -O https://raw.githubusercontent.com/terraform-google-modules/terraform-google-kubernetes-engine/v5.0.0/helpers/migrate.py
+    chmod +x migrate.py
+    ```
+
+2. Execute the migration script
+
+    ```
+    ./migrate.py
+    ```
+
+    Output will be similar to the following:
+    ```
+    ---- Migrating the following modules:
+    -- module.gke-cluster-dev.module.gke
+    ---- Commands to run:
+    terraform state mv -state terraform.tfstate "module.gke-cluster-dev.module.gke.google_container_cluster.zonal_primary[0]" "module.gke-cluster-dev.module.gke.google_container_cluster.primary[0]"
+    terraform state mv "module.gke-cluster-dev.module.gke.google_container_node_pool.zonal_pools[0]" "module.gke-cluster-dev.module.gke.google_container_node_pool.pools[0]"
+    ```
+
+3. Execute the provided state migration commands (backups are automatically created).
+
+4. Run `terraform plan` to confirm no changes are expected.
+
 ### Service Account creation
 
 Previously, if you explicitly specified a Service Account using the `service_account` variable on the module this was sufficient to force that Service Account to be used.