Decouple Readme based on use cases

I'm not tied to filenames, but from your response we appear to be in agreement that this README should be a landing page and not a catalogue of all the different sets of knowledge required to use the files in this repo.

Extrapolating from your thoughts, I think we need:

- README.md with sections briefly explaining the repo and the different types of uses cases. Each with links to get users/contributors where they need to go.
- USAGE.md document outlining expected ways to consume these files
- CONTRIBUTING.md document outlining what content we accept.

_Originally posted by @jpogran in https://github.com/hashicorp/syntax/pull/11#discussion_r823770761_