diff --git a/pkgs/test/nixos-functions/default.nix b/pkgs/test/nixos-functions/default.nix
index a53157066bb..c8f7122006f 100644
--- a/pkgs/test/nixos-functions/default.nix
+++ b/pkgs/test/nixos-functions/default.nix
@@ -1,3 +1,14 @@
+/*
+
+This file is a test that makes sure that the `pkgs.nixos` and
+`pkgs.nixosTest` functions work. It's far from a perfect test suite,
+but better than not checking them at all on hydra.
+
+To run this test:
+
+    nixpkgs$ nix-build -A tests.nixos-functions
+
+ */
 { pkgs, lib, stdenv, ... }:
 
 lib.optionalAttrs stdenv.hostPlatform.isLinux (