lib/categories.nix: create

As a proof-of-concept for NixOS/rfcs#146

lib/categories.nix

@@ -0,0 +1,58 @@
+/* List of categories
+    ```nix
+    handle = {
+      # Required
+      name = "Category Name";
+      description = ''
+        Description of category
+      '';
+      relatedCategories = [ related01 related02 . . . ];
+    };
+    ```
+
+    where
+
+    - `handle` is the handle you are going to use in nixpkgs expressions;
+       usually the same as the name but in camelCase
+    - `name` is the category name
+    - `description` is a description of the category
+    - `relatedCategories` is a (possibly empty) list of related categories (like
+      the `handle` above)
+
+    More fields may be added in the future.
+
+    When editing this file:
+     * do not modify any of
+         - mainCategories
+         - additionalCategories
+         - reservedCategories
+       except by reasons of force majeure (e.g. an update on the Freedesktop.org
+       Desktop Menu Specification);
+     * keep the lists alphabetically sorted;
+     * test the validity of the format with:
+         nix-build lib/tests/categories.nix
+*/
+{ lib }:
+
+let
+  mainCategories = {
+  };
+
+  additionalCategories = {
+  };
+
+  reservedCategories = {
+  };
+
+  # "Custom" list of categories for the use of Nixpkgs
+  nixpkgsAdditionalCategories = {
+  };
+
+  allCategories = lib.foldl lib.recursiveUpdate {} [
+    mainCategories
+    additionalCategories
+    reservedCategories
+    nixpkgsAdditionalCategories
+  ];
+in
+allCategories