ref-manual: Added rootfs-postcommands class description.

(From yocto-docs rev: 237f8c8fbefc0ad753d47d891b24a49e0e0c6b5e)

Signed-off-by: Scott Rifenbark <srifenbark@gmail.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>

documentation/ref-manual/ref-classes.xml

@@ -2983,6 +2983,10 @@
         The <filename>rootfs*</filename> classes support creating
         the root filesystem for an image and consist of the following classes:
         <itemizedlist>
+            <listitem><para>
+                The <filename>rootfs-postcommands</filename> class, which
+                defines filesystem post-processing functions for image recipes.
+                </para></listitem>
             <listitem><para>
                 The <filename>rootfs_deb</filename> class, which supports
                 creation of root filesystems for images built using