chore: Add v10.0 upgrade doc (#599)

Author: morgante

docs/upgrading_to_v10.0.md

@@ -0,0 +1,50 @@
+# Upgrading to v10.0
+
+The v10.0 release of *kubernetes-engine* is a backwards incompatible
+release.
+
+## Provider Version
+
+- For beta modules, support for Google provider versions older than v3.29 has been removed.
+- For GA modules, provider version constraints are unchanged.
+
+## Default Machine Type
+The new default machine type for node pools is now `e2-medium`. These VMs are more cost-effective. e2-medium machine types sustain 2 vCPUs at 50% of CPU time, yielding the same processing capacity as 1 vCPU and can temporarily burst to 2 vCPUs. Thus the sustained throughput is the same as n1-standard-1, so long as there are multiple threads running on the node -- and if you have multiple containers -- this is almost certainly the case.
+
+However, `e2-medium` nodes don't work for all use cases. In particular, if you are using any of these features you should choose another machine type:
+
+1. You're running single-threaded applications with sensitive performance requirements.
+2. You use GPUs or local SSDs or sole tenancy, all of which are unsupported by the new default machine type e2-medium.
+
+If you want to keep the old node type (`n1-standard-2`), you should specify it explicitly:
+
+```diff
+module "gke" {
+    source        = "terraform-google-modules/kubernetes-engine/google"
+-   version       = "~> 9.0"
++   version       = "~> 10.0"
+...
+    node_pools = [
+      {
+        name         = "pool-01"
++       machine_type = "n1-standard-2"
+      },
+    ]
+}
+```
+
+## Pod Security Policy
+The interface for configuring pod security policy has been [simplified](https://github.com/terraform-google-modules/terraform-google-kubernetes-engine/commit/6069ece9cd12acbbba8ff16ab0cbc9b17bc47985) to a simple boolean. If you've changed the default, you'll need to update your config:
+
+```diff
+module "gke" {
+    source        = "terraform-google-modules/kubernetes-engine/google"
+-   version       = "~> 9.0"
++   version       = "~> 10.0"
+...
+-   pod_security_policy_config = [{
+-     enabled = true
+-   }]
++   enable_pod_security_policy = true
+}
+```