doc: add users to mdbook

Author: nrdxp

doc/src/SUMMARY.md

@@ -8,3 +8,4 @@
   - [Hosts](./layout/hosts.md)
   - [Profiles](./layout/profiles.md)
   - [Suites](./layout/suites.md)
+  - [Users](./layout/users.md)

doc/src/layout/users.md

@@ -0,0 +1,53 @@
+# Users
+
+Users are a special case of [profiles](./profiles.md) that define system users
+and [home-manager][home-manager] configurations. For your convenience, home
+manager is wired in by default so all you have to worry about is declaring your
+users. For a fully fleshed out example, check out the developers personal
+[branch](https://github.com/nrdxp/nixflk/tree/nrd/users/nrd/default.nix).
+
+## Basic Usage
+`users/myuser/default.nix`:
+```nix
+{ ... }:
+{
+  users.users.myuser = {
+    isNormalUser = true;
+  };
+
+  home-manager.users.myuser = {
+    programs.mpv.enable = true;
+  };
+}
+
+```
+
+## External Usage
+You can easily use the defined home-manager configurations outside of NixOS
+using the `hmActivations` meta-package defined in the flakes `legacyPackages`
+output. The [flk](../flk/flk.md) helper script makes this even easier. Just
+make sure you have home-manager
+[installed](https://nix-community.github.io/home-manager/index.html#ch-installation).
+
+This is great for keeping your environment consistent across Unix systems,
+including OSX.
+
+### From within the projects devshell:
+```sh
+# builds the nixos user defined in the NixOS host
+flk home NixOS nixos
+
+# activate
+flk home NixOS nixos switch
+```
+
+### Manually from outside the project:
+```sh
+# build
+nix build "github:nrdxp/nixflk#hmActivationPackages.NixOS.nixos"
+
+# activate
+./result/activate && unlink result
+```
+
+[home-manager]: https://nix-community.github.io/home-manager

users/README.md

@@ -0,0 +1 @@
+../doc/src/layout/users.md