nixos/state: initialize requirements document

[bendlas]

Description of changes

Sketch out possible requirements document for a holistic approach towards state management in nixos.

This branch is subject to rebase on NixOS/nixpkgs/master.

This branch is in my fork for now, but if there's interest, I'm happy to maintain it within NixOS, in order to allow maintainers to directly write to the documents.

Things done

• Create a requirements document, check out rendered version: https://github.com/bendlas/nixpkgs/blob/nixos-state-management/nixos/state/requirements.md

References

• RFC for ensure-style options
• RFC for NixOS migrations

FAQ

Q: Why can this work?
A: Because it doesn't try to invent anything new, but instead work with what's there.

Q: Is this guaranteed to work?
A: No, there are still plenty of hairy details, see document.

[nixos-discourse]

This pull request has been mentioned on NixOS Discourse. There might be relevant details there:

https://discourse.nixos.org/t/can-we-please-stop-breaking-stuff-willy-nilly/48496/58