update readme to include expected output on `terraform init -reconfigure`

[stavxyz]

When terraform init -reconfigure is run, we can let the user know they can expect the following for a brand new project:

Initializing the backend...
Do you want to copy existing state to the new backend?
  Pre-existing state was found while migrating the previous "local" backend to the
  newly configured "s3" backend. No existing state was found in the newly
  configured "s3" backend. Do you want to copy this state to the new "s3"
  backend? Enter "yes" to copy and "no" to start with an empty state.

  Enter a value: yes