note about this directory.

nix/README

+This directory contains expressions for packaging PaymentServer on NixOS.
+The contents of this directory are almost entirely automatically generated by stack-to-nix.
+Editing them is probably a bad idea since it will make future automated updates more difficult.
+
+However, I did edit default.nix to replace the master tarball of haskell.nix with a particular revision.
+Read the comment there for details.
+
+See https://github.com/input-output-hk/haskell.nix for details of stack-to-nix.