callPackages and readme

callPackage/default.nix

@@ -0,0 +1,7 @@
+let 
+  pkgs = import <nixpkgs> { };
+in 
+rec{
+  hello = pkgs.callPackage ./hello.nix { audience = "people"; };
+  hello-folks = hello.override { audience = "folks"; };
+}

callPackage/hello.nix

@@ -0,0 +1,7 @@
+{ 
+  writeShellScriptBin,
+  audience ? "world",
+}:
+writeShellScriptBin "hello" ''
+  echo "Hello, ${audience}!"
+''