ref-manual: Added variable description for ROOTFS_POSTPROCESS_COMMAND.

(From yocto-docs rev: ad003d9f28d50ce66dc717f7a67710c868a3b892)

Signed-off-by: Scott Rifenbark <scott.m.rifenbark@intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>

documentation/ref-manual/ref-variables.xml

@@ -4152,6 +4152,20 @@ recipes-graphics/xorg-font/font-alias_1.0.3.bb:PR = "${INC_PR}.3"
             </glossdef>
         </glossentry>
 
+        <glossentry id='var-ROOTFS_POSTPROCESS_COMMAND'><glossterm>ROOTFS_POSTPROCESS_COMMAND</glossterm>
+            <glossdef>
+                <para>
+                    Added by classes to run post processing commands once the 
+                    OpenEmbedded build system has created the root filesystem.
+                    You can specify shell commands and, if necessary, the path
+                    to the root filesystem image:
+                    <literallayout class='monospaced'>
+     ROOTFS_POSTPROCESS_COMMAND += "${IMAGE_ROOTFS}&lt;shell_command&gt;; ... "
+                    </literallayout>
+                </para>
+            </glossdef>
+        </glossentry>
+
         <glossentry id='var-RPROVIDES'><glossterm>RPROVIDES</glossterm>
             <glossdef>
                 <para>