Add update-nixpkgs / diff-closures documentation

Adding the commands from !259 (merged) to the docs else I forget how to use it.

docs/dev/README.rst

@@ -51,11 +51,22 @@ To update the version of NixOS we deploy with, run:
 
    nix-shell --run 'update-nixpkgs'
 
-That will update ``nixpkgs-2015.json`` to the latest release on the nixos-21.05 channel.
+That will update ``nixpkgs.json`` to the latest release on the nixos release channel.
 
 To update the channel, the script will need to be updated,
 along with the filenames that have the channel in them.
 
+To create a text summary of what an update changes - to put in Merge Requests, for example - run:
+
+.. code: shell
+
+    nix-build -A morph -o result-before
+    update-nixpkgs
+    nix-build -A morph -o result-after
+    nix-shell -p nixUnstable
+    nix --extra-experimental-features nix-command store diff-closures ./result-before/ ./result-after/
+
+
 Gitlab Repositories
 ```````````````````
 To update the version of packages we import from gitlab, run: