fix: add upgrade notes for v36

docs/upgrading_to_v36.0.md

@@ -0,0 +1,19 @@
+# Upgrading to v36.0
+
+The v36.0 release of _kubernetes-engine_ is a backwards incompatible release.
+
+### master_authorized_networks default value
+
+The default value for `master_authorized_networks` has been changed from `[]` to `null`. To maintain the previous default behavior, set `master_authorized_networks` to `[]`. This change is because the API interprets an existing `master_authorized_networks_config` to enable authorized networks, but we want to be explicit about enabling this option.
+
+```
+  module "gke" {
+    source  = "terraform-google-modules/kubernetes-engine/google//modules/private-cluster"
+    version = "~> 36.0"
+
+    project_id  = var.project_id
+    name        = var.cluster_name
+
++   master_authorized_networks = []
+  }
+```