nixos/README.md: relax the requirement of providing option defaults

If we enforce the requirement of option defaults to the extreme, then we end up with silly sentinel values, a bunch of nullable types, and assertions checking for sentinel values.

nixos/README.md

@@ -83,6 +83,9 @@ Reviewing process:
 - 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 it in order to use the service.
+      2. The lack of a default does not break evaluation when the 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`.