Add a new section: "How to rebase on a supported FreeBSD branch".

While here, make some tricky markup a lot clearer.

en_US.ISO8859-1/articles/portbuild/article.sgml

@@ -2121,7 +2121,7 @@ ssh_cmd="/usr/local/bin/ssh"
 	  <listitem>
 	    <para>Add
 	      <makevar>SRC_BRANCH_<replaceable>new-branch</replaceable>_TAG</makevar>
-	      <literal>=.</literal>.</para>
+	      <literal>=.</literal> (literal period).</para>
 	  </listitem>
 	</itemizedlist>
       </listitem>
@@ -2242,6 +2242,63 @@ zfs destroy -r a/snap/src-<replaceable>old-branch</replaceable></command></para>
     </itemizedlist>
   </sect1>
 
+  <sect1 id="rebase-branch">
+    <title>How to rebase on a supported &os; branch</title>
+
+    <para>As of 2011, the philosophy of package building is to build
+      packages based on <emphasis>the earliest supported release</emphasis>
+      of each branch.  e.g.: if on <literal>RELENG-8</literal>, the
+      following releases are supported: 8.1, 8.2, 8.3; then
+      <literal>packages-8-stable</literal> should be built from 8.1.</para>
+
+    <para>As releases go End-Of-Life (see
+      <ulink url="http://www.freebsd.org/security/index.html#supported-branches">chart</ulink>),
+      a full (not incremental!) package build should be done and uploaded.
+      </para>
+
+    <para>The procedure for the new codebase is as follows:</para>
+
+    <itemizedlist>
+      <listitem>
+	<para>Edit <filename>/var/portbuild/conf/server.conf</filename>
+	  with the following changes:</para>
+	  <itemizedlist>
+	    <listitem>
+	      <para>Change
+	        <makevar>SRC_BRANCH_<replaceable>branch</replaceable>_TAG</makevar> to
+	        <literal>RELENG_<replaceable>branch</replaceable>_<replaceable>N</replaceable></literal>
+	        where <literal>N</literal> is the newest 'oldest' release
+	        for that branch.</para>
+	    </listitem>
+	  </itemizedlist>
+      </listitem>
+
+      <listitem>
+	<para>Run <command>
+/var/portbuild/updatesnap</command> manually.</para>
+      </listitem>
+
+      <listitem>
+	<para>Run <command>
+dopackages</command> with <literal>-nobuild</literal>.</para>
+      </listitem>
+
+      <listitem>
+	<para>Follow the <link linkend="setup">setup procedure</link>.</para>
+      </listitem>
+
+      <listitem>
+	<para>Now you can run <command>
+dopackages</command> without <literal>-nobuild</literal>.</para>
+      </listitem>
+
+    </itemizedlist>
+
+    <para>The procedure for the old codebase is left as an
+      exercise for the reader.</para>
+
+  </sect1>
+
   <sect1 id="new-arch">
     <title>How to configure a new architecture</title>