Add documentation for NixOS integration.

docs/nixos.md

@@ -0,0 +1,50 @@
+# NixOS Integration
+
+This repo can be used as a [flake](https://nixos.wiki/wiki/Flakes), and provides `overlay` and `nixosModule` outputs. This functionality can also be used without flakes.
+
+## Configuring overlay
+
+If you just want to use ROS packages in your NixOS configuration, you may add this repo as a nixpkgs overlay. If you want more complete integration of ROS with NixOS, you may instead use the provided NixOS module (see next section).
+
+Note that this overlay applies overrides to the Python package set, and therefore cannot coexist with other overlays that do the same, due to limitations in nixpkgs.
+
+If using flakes, first add this repo as a flake input and then use the overlay by adding the following config snippet:
+```nix
+nixpkgs.overlays = [ nix-ros-overlay.overlay ];
+```
+
+If not using flakes, add the following config snippet, filling out the commit and sha256 as needed:
+```nix
+nixpkgs.overlays = let
+    nix-ros-overlay = builtins.fetchTarball {
+      url = https://github.com/lopsided98/nix-ros-overlay/archive/<commit>.tar.gz;
+      sha256 = "<sha256>";
+    };
+in [ (import (nix-ros-overlay + "/overlay.nix")) ];
+```
+
+## Configuring NixOS module
+
+This repo includes a NixOS module that integrates ROS into NixOS. It adds the package set as a overlay, configures ROS environment variables and may optionally start ROS nodes as systemd services.
+
+If using flakes, import the `nixosModule` output in your config:
+```nix
+imports = [ nix-ros-overlay.nixosModule ];
+```
+
+Otherwise, import the `modules` directory:
+```nix
+imports = let
+    nix-ros-overlay = builtins.fetchTarball {
+      url = https://github.com/lopsided98/nix-ros-overlay/archive/<commit>.tar.gz;
+      sha256 = "<sha256>";
+    };
+in [ (nix-ros-overlay + "/modules") ];
+```
+
+Then, enable basic ROS integration:
+```nix
+services.ros.enable = true;
+```
+
+See the contents of the `modules` directory for the other available options.