Add a note about project structure for Terraform + Ansible to README

[nbering]

I've made some assumptions in the design that prefer a project were Terraform and Ansible configuration are in the same directory, because that's how I organize things.

I've also provided a mechanism for configuring the script to have a separate Terraform directory. It's documented under Environment Variables, but something clearer under Usage, or a separate section on project structure would probably be better.