Add a nixos module for the ZKAP Spending Service

Part of #84.

docs/source/dev/README.rst

@@ -42,6 +42,21 @@ Consult the `official documentation on NixOS Tests <https://nixos.org/manual/nix
 Updatings Pins
 --------------
 
+Nixpkgs
+```````
+
+To update the version of NixOS we deploy with, run:
+
+.. code: shell
+
+   nix-shell --run 'update-nixpkgs'
+
+That will update ``nixpkgs-2015.json`` to the latest release on the nixos-21.05 channel.
+
+To update the channel, the script will need to be updated,
+along with the filenames that have the channel in them.
+
+
 Gitlab Repositories
 ```````````````````
 To update the version of packages we import from gitlab, run: