Add warning about the sparse population of /etc/shorewall under Debian

Signed-off-by: Tom Eastep <teastep@shorewall.net>

docs/configuration_file_basics.xml

@@ -306,6 +306,24 @@
     <para><emphasis role="bold">If you need to change a file in
     /usr/share/shorewall/, copy it to <filename>/etc/shorewall</filename> and
     modify the copy</emphasis></para>
+
+    <warning>
+      <para><emphasis role="bold">Note to Debian and Ubuntu
+      Users</emphasis></para>
+
+      <para>If you install using the .deb or using the tarball installers with
+      the default <filename>shorewallrc.debian</filename> file, you will find
+      that your <filename class="directory">/etc/shorewall</filename>
+      directory is practially empty. This is intentional. When you install the
+      .deb, the released configuration file skeletons may be found on your
+      system in the directory <filename
+      class="directory">/usr/share/doc/shorewall/default-config</filename>. If
+      you install using the tarball installers, the files are in the
+      <filename>/usr/share/shorewall/configfiles</filename> sub-directory.
+      Simply copy the files you need from the appropriate directory to
+      <filename class="directory">/etc/shorewall</filename> and modify the
+      copies.</para>
+    </warning>
   </section>
 
   <section id="Manpages">