nixos/manual: Re-add development section.

I think this has been accidentally dropped by a099ca4, at least there is
no reason stated, why it shouldn't be included, so I'm bringing it back.

Signed-off-by: aszlig <aszlig@redmoonstudios.org>

nixos/doc/manual/manual.xml

@@ -32,6 +32,7 @@
   <xi:include href="configuration/configuration.xml" />
   <xi:include href="administration/running.xml" />
   <!-- <xi:include href="userconfiguration.xml" /> -->
+  <xi:include href="development/development.xml" />
   <xi:include href="release-notes/release-notes.xml" />
 
   <appendix xml:id="ch-options">