Update README.md

Co-authored-by: Paul Hirsch <59626817+pauldoomgov@users.noreply.github.com>

README.md

@@ -147,6 +147,10 @@ module "egress_space" {
 
 ## Testing
 
+
+> **Warning**
+Tests provision resources in the real world when not using `mock_provider`! Take care that `CF_USER`/`CF_PASSWORD` are set to an account in a suitable non-production space. If other providers, such as the AWS provider, are used, ensure the same care is taken with their credentials in your shell before running `terraform test`.
+
 [Terraform tests](https://developer.hashicorp.com/terraform/language/tests) are in progress of being written. To run for any module with a `tests` directory:
 
 1. Set `CF_USER` and `CF_PASSWORD` env variables with SpaceDeployer credentials that can access the space(s) being used for tests