Include explicit information about adjusting `modules` directory structure and `lib.<namespace>` requirements in migration docs

[dukeofcool199]

Please include explicit documentation in the migration docs that include information about the following facts

• when migrating to v2 you must move all of you existing modules into one of the nixos/home/darwin directories
  • I know this is implied from the rest of the docs, but I got bitten by this personally
• that all references to custom lib attributes and custom package must be referenced by the custom namespace set in snowfall.namespace

[dukeofcool199]

I would be willing to update the docs myself if you are accepting PRs in this repo

[jakehamilton]

Ah, I agree we should call these out directly. If you want to draft a PR, I'd be happy to review and merge it!

[wthueb]

Not sure where to ask this, so asking it here. Is it not possible to have modules for nixos and darwin without just duplicating them?