Use different AWS accounts (per stage)

Hi, firstly amazing work, thanks for open sourcing it! 😎

Early on in the README the [**Concepts** ](https://github.com/FormidableLabs/terraform-aws-serverless#concepts) section states:

> This module allows practical isolation / compartmentalization of privileges within a single AWS account along the following axes:

It's a known practice to separate stages in to different AWS accounts, e.g:

* From the [Serverless docs](https://serverless.com/framework/docs/providers/aws/guide/credentials/#per-stage-profiles)
> As an advanced use-case, you can deploy different stages to different accounts by using different profiles per stage.

* From [a top Google result](https://serverless-stack.com/chapters/stages-in-serverless-framework.html):
> Most companies don’t keep their production infrastructure in the same account as their development infrastructure. This helps reduce any cases where developers accidentally edit/delete production resources.

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It'd be great if you could indicate in the README:

1. Why you decided to implement a single AWS account solution here, and:
1. Whether there are plans (or willingness to accept work on) adding support for using different AWS accounts (per stage).