Add some text describing the "include" kernel configuration directive,

and give an example.  In practice, including GENERIC and then simply
providing a few additional options or devices is the preferred way to
configure kernels now, rather than writing them from scratch (except
in embedded environments).  This text could probably use some
refinement.

Suggested by:	Richard Clayton <richard.clayton at cl.cam.ac.uk>

en_US.ISO8859-1/books/handbook/kernelconfig/chapter.sgml

@@ -497,6 +497,34 @@ following line in &man.loader.conf.5:
       architecture independent options, see
       <filename>/usr/src/sys/conf/NOTES</filename>.</para>
 
+    <para>As of &os; 5.0, a new <literal>include</literal> directive is
+      available for use in configuration files.  This allows another
+      configuration file to be logically included in the current one, making
+      it easy to maintain small changes relative to an existing file.  For
+      example, if you require a <filename>GENERIC</filename> kernel with
+      only a small number of additional options or drivers, this allows you
+      to maintain only a delta with respect to GENERIC:</para>
+
+    <programlisting>include GENERIC
+ident MYKERNEL
+
+options         IPFIREWALL
+options         DUMMYNET
+options         IPFIREWALL_DEFAULT_TO_ACCEPT
+options         IPDIVERT
+</programlisting>
+
+    <para>Many administrators will find that this model offers significant
+      benefits over the historic writing of configuration files from scratch:
+      the local configuration file will express only local differences from
+      a <filename>GENERIC</filename> kernel and as upgrades are performed,
+      new features added to <filename>GENERIC</filename> will be added to the
+      local kernel unless specifically prevented using
+      <literal>nooptions</literal> or <literal>nodevice</literal>.  The
+      remainder of this chapter addresses the contents of a typical
+      configuration file and the role various options and devices
+      play.</para>
+
     <note>
       <para>To build a file which contains all available options,
 	as normally done for testing purposes, run the following