Contributor documentation for Modular Services

[roberth]

1. Anyone should be able to review this new kind of module
2. Modular Services is a new feature for an unmerged RFC: manage expectations and avoid imposing any undue load on maintainers; try to help them instead

Things done

• Built on platform:
  • x86_64-linux
  • aarch64-linux
  • x86_64-darwin
  • aarch64-darwin
• Tested, as applicable:
  • NixOS tests in nixos/tests.
  • Package tests at passthru.tests.
  • Tests in lib/tests or pkgs/test for functions and "core" functionality.
• Ran nixpkgs-review on this PR. See nixpkgs-review usage.
• Tested basic functionality of all binary files, usually in ./result/bin/.
• Nixpkgs Release Notes
  • Package update: when the change is major or breaking.
• NixOS Release Notes
  • Module addition: when adding a new NixOS module.
  • Module update: when the change is significant.
• Fits CONTRIBUTING.md, pkgs/README.md, maintainers/README.md and other READMEs.

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[aanderse]

i like the content of this, it is nice and consistent with the discussion in #430490, manages expectations of all potentially involved parties 👍

thanks for doing this @roberth ❤️