documentation/poky-ref-manual/extendpoky.xml: YOCTO #1104 EXTRA_IMAGE_FEATURES

YOCTO #1104: The section that describes how to customize images
with new features failed to mention the variable EXTRA_IMAGE_FEATURES.
I added text to include that option and referenced the variable.
(From yocto-docs rev: 69113aeebe4b7047c18727d07d134560ae2018c5)

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

documentation/poky-ref-manual/extendpoky.xml

@@ -349,9 +349,9 @@ RRECOMMENDS_task-custom-tools = "\
         </section>
 
         <section id='usingpoky-extend-customimage-imagefeatures'>
-            <title>Customizing Images Using Custom IMAGE_FEATURES</title>
+            <title>Customizing Images Using Custom IMAGE_FEATURES and EXTRA_IMAGE_FEATURES</title>
             <para>
-                Ultimately users might want to add extra image "features" as used by Poky with the 
+                Ultimately users might want to add extra image "features" to the set used by Poky with the 
                 <glossterm><link linkend='var-IMAGE_FEATURES'>IMAGE_FEATURES</link></glossterm>
                 variable. 
                 To create these features, the best reference is 
@@ -363,6 +363,9 @@ RRECOMMENDS_task-custom-tools = "\
                 </glossterm> variable is generated automatically. 
                 Users can add extra features by extending the class or creating a custom class for use 
                 with specialized image <filename>.bb</filename> files.
+                You can also add more features by configuring the 
+                <glossterm><link linkend='var-EXTRA_IMAGE_FEATURES'>EXTRA_IMAGE_FEATURES</link></glossterm>
+                variable in the <filename>local.conf</filename> file.
             </para>
             <para>
                 Poky ships with two SSH servers you can use in your images: Dropbear and OpenSSH.