nixos/doc: Mention how to contribute to it

Instead of putting a README in the directory where the manual is
written, put the information from it straight in the docs themselves.
It's a bit untrivial to guess the manual is located exactly there for
contributers.

nixos/doc/manual/README

@@ -1,12 +1,3 @@
-To build the manual, you need Nix installed on your system (no need
-for NixOS). To install Nix, follow the instructions at
+Moved to: ./contributing-to-this-manual.xml. Link:
 
-    https://nixos.org/nix/download.html
-
-When you have Nix on your system, in the root directory of the project
-(i.e., `nixpkgs`), run:
-
-    nix-build nixos/release.nix -A manual.x86_64-linux
-
-When this command successfully finishes, it will tell you where the
-manual got generated.
+https://nixos.org/manual/nixos/unstable/#chap-contributing

nixos/doc/manual/contributing-to-this-manual.xml

@@ -0,0 +1,22 @@
+<chapter xmlns="http://docbook.org/ns/docbook"
+         xmlns:xlink="http://www.w3.org/1999/xlink"
+         xml:id="chap-contributing">
+ <title>Contributing to this documentation</title>
+ <para>
+  The DocBook sources of NixOS' manual are in the <filename
+xlink:href="https://github.com/NixOS/nixpkgs/tree/master/nixos/doc/manual">
+nixos/doc/manual</filename> subdirectory of the <link
+xlink:href="https://github.com/NixOS/nixpkgs">Nixpkgs</link> repository.
+ </para>
+ <para>
+  You can quickly check your edits with the following:
+ </para>
+<screen>
+<prompt>$ </prompt>cd /path/to/nixpkgs/nixos/doc/manual
+<prompt>$ </prompt>nix-build nixos/release.nix -A manual.x86_64-linux
+</screen>
+ <para>
+  If the build succeeds, the manual will be in
+  <filename>./result/share/doc/nixos/index.html</filename>.
+ </para>
+</chapter>

nixos/doc/manual/manual.xml

@@ -19,5 +19,6 @@
   <xi:include href="./generated/options-db.xml"
                 xpointer="configuration-variable-list" />
  </appendix>
+ <xi:include href="contributing-to-this-manual.xml" />
  <xi:include href="release-notes/release-notes.xml" />
 </book>