Add documentation for importing existing property logic

[aweingarten]

User Story

As a customer with an existing terraform property I want clear guidance for manage my existingproperty with this starterkit and terraform.

Implementation Details

Update the docs to show how

1. export the config as terraform
2. set the terraform state for production
3. update the tf in repo to use the exported terraform.

Acceptance Criteria

A team member can follow the instructions and apply this to an existing project