chore: Add upgrade guide for v8.0 (#401)

BREAKING CHANGE: Using the gcloud module now requires `curl` to be installed. See the upgrade guide for details.

Author: morgante

docs/upgrading_to_project_factory_v8.0.md

@@ -0,0 +1,9 @@
+# Upgrading to Project Factory v8.0
+
+The v8.0 release of Project Factory updates the `gcloud` module to use the [1.0.0](https://github.com/terraform-google-modules/terraform-google-gcloud/blob/master/CHANGELOG.md#100-2020-04-15) version.
+
+## gcloud module
+If you are relying on the built-in gcloud module, you will need to make sure `curl`
+is available in your Terraform execution environment.
+
+If you have `skip_download` set to `true`, no change is necessary.