docs: describe tf state options in /setup/README.md

[davidstanke]

Currently, the setup instructions store TF state locally on the user's local machine. Let's provide some instructions on how to use a remote state (e.g. gcs), which is better for ongoing maintenance.

[davidstanke]

backend.tf is already excluded by .gitignore, so we can instruct users to add a file of that name.

[fbtravi]

hello, a while ago I had an idea similar to this, but I believe it was not in the project plans.

If you want I can return

#414