establish flakeArg construct & add docu

flake.nix

@@ -42,8 +42,17 @@
     { self, ... }@inputs:
     let
       inherit (self) outputs;
+      # every flake "submodule" gets this passed:
       flakeArg = {
-        inherit self inputs outputs;
+        # Usage in submodule:
+        # { ... }@flakeArg: { }
+        # add "..." this so new ones can easily be added
+        inherit
+          # flake refs
+          self # reflection
+          inputs # evaluated inputs
+          outputs # evaluated outputs
+          ;
       };
       # constants
       system = "x86_64-linux";

nix/README.md

@@ -1,3 +1,5 @@
+# nix configs & modules
+
 This directory contains all nix-related (esp. NixOS) files.
 
 Normally, I would have placed these directories on the same level as flake.nix, but:
@@ -7,3 +9,8 @@ Normally, I would have placed these directories on the same level as flake.nix,
   (i.e. also including systems without NixOS)
   it also contains files for different systems.
   These should be separated from nix files, in general.
+
+
+## flakeArg
+
+Every flake "submodule" gets passed a `flakeArg` parameter to access the flake’s inputs & outputs and other goodies. This is documented in ../flake.nix.