Add update documentation

docs/Update.md

@@ -0,0 +1,31 @@
+# Update
+
+The following document describes the process on updating the NixOS system.
+
+## Updating Flake
+
+The flake's lockfile will need to be updated to fetch newer packages. This is
+like running `apt update` on a Debian system.
+
+Run the following command in the flake git repository:
+
+```bash
+nix flake update
+```
+
+> **NOTE:** This will update the git repository, but might be a good idea to
+check if the changes break the system at all before committing them to the
+repository, or at least the `main` branch.
+
+## Rebuilding System
+
+You then need to reapply the flake to the system for the changes to take
+affect. This is like running `apt upgrade` on a Debian system.
+
+Run the following command in the flake git repository, replacing `#HOSTNAME`
+with the current system's hostname:
+
+```bash
+sudo nixos-rebuild switch --flake .#HOSTNAME
+```
+