Add a nixos module for the ZKAP Spending Service

Part of #84.

docs/source/dev/README.rst

@@ -57,6 +57,20 @@ 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:
+
+.. code: shell
+
+   nix-shell --command 'tools/update-gitlab nixos/pkgs/<package>/repo.json'
+
+That will update the package to point at the latest version of the project.\
+The command uses branch and repository owner specified in the ``repo.json`` file,
+but you can override them by passing the ``--branch`` or ``-owner`` arguments to the command.
+A specific revision can also be pinned, by passing ``-rev``.
+
+
 Architecture overview
 ---------------------