nixos/README.md: relax the requirement of providing option defaults (#334509)

- Ensure that the introduced options are correct.

  - Type should be appropriate (string related types differs in their merging capabilities, `loaOf` and `string` types are deprecated).

  - Description, default and example should be provided.

    - Defaults may only be omitted if both:

      1. The user is required to set the default in order to properly use the service.

      2. The lack of a default does not break evaluation when the module is not enabled.

- Ensure that module `meta` field is present

  - Maintainers should be declared in `meta.maintainers`.

  - Module documentation should be declared with `meta.doc`.