This repository is obsolete

Add a warning that the NixOS repo has been merged into Nixpkgs.

Author: edolstra

README.md

@@ -0,0 +1,6 @@
+# The NixOS repository is obsolete!
+
+The NixOS source tree has been [merged](https://github.com/NixOS/nixpkgs/issues/1041) into the Nixpkgs 
+repository, where it now lives in the [`nixos` subdirectory](https://github.com/NixOS/nixpkgs/tree/master/nixos).
+Please file NixOS bugs in the [Nixpkgs issue tracker](https://github.com/NixOS/nixpkgs/issues) using the
+`nixos` label.