modules/README: init

Explain the purpose and usage of this new directory.

Changed files
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modules
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modules/README.md
··· 1 + # `<nixpkgs>/modules` 2 + 3 + This directory hosts subdirectories representing each module [class](https://nixos.org/manual/nixpkgs/stable/#module-system-lib-evalModules-param-class) for which the `nixpkgs` repository has user-importable modules. 4 + 5 + Exceptions: 6 + - `_class = "nixos";` modules go in the `<nixpkgs>/nixos/modules` tree 7 + - modules whose only purpose is to test code in this repository 8 + 9 + The emphasis is on _importable_ modules, i.e. ones that aren't inherent to and built into the Module System application.