Start v6.0 upgrade guide

Author: morgante

docs/upgrading_to_v6.0.md

@@ -0,0 +1,32 @@
+# Upgrading to v6.0
+
+The v6.0 release of *kubernetes-engine* is a backwards incompatible
+release.
+
+## Migration Instructions
+
+### Master Authorized Networks
+Previously, setting up master authorized networks required setting a nested config within `master_authorized_networks_config`.
+Now, to set up master authorized networks you can simply pass a list of authorized networks.
+
+```diff
+ module "kubernetes_engine_private_cluster" {
+   source  = "terraform-google-modules/kubernetes-engine/google"
+-  version = "~> 5.0"
++  version = "~> 6.0"
+
+-  master_authorized_networks_config = [
++  master_authorized_networks = [
+     {
+-      cidr_blocks = [
+-        {
+-          cidr_block   = data.google_compute_subnetwork.subnetwork.ip_cidr_range
+-          display_name = "VPC"
+-        },
+-      ]
++      cidr_block   = data.google_compute_subnetwork.subnetwork.ip_cidr_range
++      display_name = "VPC"
+     },
+   ]
+ }
+```