chore: Add upgrade guide for v8.0 (#476)

* upgrade guide 8.0

* only include breaking changes

docs/upgrading_to_v8.0.md

@@ -0,0 +1,10 @@
+# Upgrading to v8.0
+
+The v8.0 release of *kubernetes-engine* is a backwards incompatible
+release.
+
+## Workload Identity (beta)
+Beta clusters now have Workload Identity enabled by default. To disable Workload Identity, set `identity_namespace = null`
+
+## Shielded Nodes (beta)
+Beta clusters now have shielded nodes enabled by default. To disable, set `enable_shielded_nodes = false`