documentation/poky-ref-manual/ref-images.xml: Added three core-images

Added descriptions for 'core-image-rt', 'core-image-rt-sdk', and
'core-image-gtk-directfb'.  These were apparently missing.

Fixes [YOCTO #1921]

Reported by Shane Wang <shane.wang@intel.com>
(From yocto-docs rev: b83f9fa2a2de867ef9973b84aa1bec94e8071e45)

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

documentation/poky-ref-manual/ref-images.xml

@@ -92,6 +92,21 @@
             See the <link linkend='platdev-appdev-external-sdk'> 
             External Development Using the Poky SDK</link> section for more information.
             </para></listitem>
+        <listitem><para><emphasis><filename>core-image-rt</filename>:</emphasis>
+            A <filename>core-image-minimal</filename> image plus a real-time test suite and 
+            tools appropriate for real-time use.</para></listitem>
+        <listitem><para><emphasis><filename>core-image-rt-sdk</filename>:</emphasis>
+            A <filename>core-image-rt</filename> image that includes everything in 
+            <filename>meta-toolchain</filename>.  
+            The image also includes development headers and libraries to form a complete 
+            stand-alone SDK.  
+            See the <link linkend='platdev-appdev-external-sdk'> External Development 
+            Using the Poky SDK</link> section for more information.</para></listitem>
+        <listitem><para><emphasis><filename>core-image-gtk-directfb</filename>:</emphasis>
+            An image that uses <filename>gtk+</filename> over <filename>directfb</filename> 
+            instead of X11.  
+            In order to build, this image requires specific distro configuration that enables 
+            <filename>gtk</filename> over <filename>directfb</filename>.</para></listitem>
     </itemizedlist>
 
     <tip>