Start drafting the upgrade guide for v5.0

Author: morgante

CHANGELOG.md

@@ -7,6 +7,10 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 Extending the adopted spec, each change should have a link to its corresponding pull request appended.
 
 ## [Unreleased]
+
+## [v5.0.0] - 2019-XX-XX
+v5.0.0 is a backwards-incompatible release. Please see the [upgrading guide](./docs/upgrading_to_v5.0.md).
+
 ### Added
 
 * Support for Intranode Visbiility (IV) and Veritical Pod Autoscaling (VPA) beta features [#216]

docs/upgrading_to_v5.0.md

@@ -0,0 +1,26 @@
+# Upgrading to v5.0
+
+The v5.0 release of *kubernetes-engine* is a backwards incompatible
+release.
+
+## Migration Instructions
+
+### 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.
+
+Now, an additional `create_service_account` has been added with a default value of `true`. If you would like to use an explicitly created Service Account from outside the module, you will need to set `create_service_account` to `false` (in addition to passing in the Service Account email).
+
+No action is needed if you use the module's default service account.
+
+```diff
+ module "kubernetes_engine_private_cluster" {
+   source  = "terraform-google-modules/kubernetes-engine/google"
+-  version = "~> 4.0"
++  version = "~> 5.0"
+
+    service_account  = "[email protected]"
++  create_service_account = true
+   # ...
+ }
+```