Added clarification for NixOS users

pills/02-install-on-your-running.xml

@@ -15,6 +15,9 @@
   <para>
     Now we'll install Nix on our running system and understand what
     changed in our system after the installation.
+    If you're using NixOS, Nix is already installed;
+    you can skip to the <link
+    linkend="install-on-your-running-system">next</link> pill.
   </para>
 
   <para>

pills/03-enter-environment.xml

@@ -325,6 +325,12 @@
 
     <screen><xi:include href="./03/channel-list.txt" parse="text" /></screen>
 
+    <para>
+      If you're using NixOS, you may not see any output from the above command
+      (if you're using the default), or you may see a channel whose
+      name begins with "nixos-" instead of "nixpkgs".
+    </para>
+
     <para>
       That's essentially the contents of <filename>~/.nix-channels</filename>.
     </para>