Add upgrade guide

docs/upgrading_to_v7.0.md

+# Upgrading to v7.0
+
+The v7.0 release of *kubernetes-engine* is a backwards incompatible
+release.
+
+## Provider Version
+Support for Google provider versions older than v3.1 has been removed due to the introduction of Surge Upgrade features.
+
+## Surge Upgrades (beta)
+In order to support [Surge Upgrades](https://cloud.google.com/kubernetes-engine/docs/concepts/cluster-upgrades#surge) on beta clusters, an `upgrade_settings` block has been added to all node pools. This will show up in `terraform plan`:
+
+```
+      + upgrade_settings {
+          + max_surge       = 1
+          + max_unavailable = 0
+        }
+```
+
+The new default behavior will upgrade 1 node at a time. You can tune this behaviour using the new `max_surge` and `max_unavailable` settings on the node pool input.
+
+Note that changing upgrade settings can be done in-place (without forcing a node pool recreation).
+
+### Node Pool for_each
+The `google_container_node_pool` resource has been updated to use `for_each` instead of `count`. This allows adding/removing node pools without causing a diff on unrelated node pools.
+
+Updating to this new format requires running a state migration. Note that this migration **must** be run with **Terraform v0.12.20**. You can use a [script](../helpers/migrate7.py) we provided to automatically make the required state migration.
+
+1. Download the script
+
+    ```sh
+    curl -O https://raw.githubusercontent.com/terraform-google-modules/terraform-google-kubernetes-engine/v7.0.0/helpers/migrate7.py
+    chmod +x migrate7.py
+    ```
+
+2. Run the script in dryrun mode to confirm the expected changes:
+
+    ```sh
+    $ ./migrate7.py --dryrun
+
+    ---- 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 migration script
+
+    ```sh
+    $ ./migrate7.py
+
+    ---- Migrating the following modules:
+    -- module.gke-cluster-dev.module.gke
+    ---- Commands to run:
+    Move "module.gke-cluster-dev.module.gke.google_container_cluster.zonal_primary[0]" to "module.gke-cluster-dev.module.gke.google_container_cluster.primary[0]"
+    Successfully moved 1 object(s).
+    Move "module.gke-cluster-dev.module.gke.google_container_node_pool.zonal_pools[0]" to "module.gke-cluster-dev.module.gke.google_container_node_pool.pools[0]"
+    Successfully moved 1 object(s).
+    Move "module.gke-cluster-dev.module.gke.null_resource.wait_for_zonal_cluster" to "module.gke-cluster-dev.module.gke.null_resource.wait_for_cluster"
+    Successfully moved 1 object(s).
+    ```
+
+4. Run `terraform plan` to confirm no changes are expected.