Add README to each module

[pacphi]

Add a README.md into each module.

The content should explain the purpose for and how to use the module. Ideally, we would explain how to consume the module without having to clone the entire Git repository, rather just the module itself.

We should also add a terraform-docs section to make it clear what are the providers, inputs, and outputs. (See https://github.com/terraform-docs/terraform-docs).