ref-manual: Provided an introductory text to the devtool ref.

I put in some introductory material for the new devtool
quick reference chapter.

(From yocto-docs rev: dc46f47692a533ac2450bf459c098435c57e6c07)

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

documentation/ref-manual/ref-devtool-reference.xml

@@ -5,14 +5,35 @@
 <chapter id='ref-devtool-reference'>
     <title><filename>devtool</filename> Quick Reference</title>
 
+    <para>
+        The <filename>devtool</filename> command is pivotal when using an
+        <ulink url='&YOCTO_DOCS_SDK_URL;#sdk-extensible'>extensible SDK</ulink>.
+        This command-line tool provides a number of features that help you
+        build, test and package software within the extensible SDK, and
+        optionally integrate it into an image built by the OpenEmbedded build
+        system.
+    </para>
+
+    <para>
+        This chapter provides a Quick Reference for the
+        <filename>devtool</filename> command.
+        For more information on how to apply the command, see the
+        "<ulink url='&YOCTO_DOCS_SDK_URL;#sdk-extensible'>Using the Extensible SDK</ulink>"
+        section in the Yocto Project Software Development Kit (SDK) Developer's
+        Guide.
+    </para>
+
     <section id='devtool-getting-help'>
         <title>Getting Help</title>
 
         <para>
-                The easiest way to get help with the
+            The <filename>devtool</filename> command line is organized
-                <filename>devtool</filename> command is using the
+            similarly to Git in that it has a number of sub-commands for
-                <filename>--help</filename> option:
+            each function.
+            You can run <filename>devtool --help</filename> to see all
+            the commands:
             <literallayout class='monospaced'>
+     $ devtool --help
      usage: devtool [--basepath BASEPATH] [--bbpath BBPATH] [-d] [-q]
                     [--color COLOR] [-h]
                     &lt;subcommand&gt; ...
@@ -546,7 +567,7 @@
             The <filename>devtool status</filename> command has no
             command-line options:
             <literallayout class='monospaced'>
-     devtool status
+     $ devtool status
             </literallayout>
             Following is sample output after using
             <ulink url='&YOCTO_DOCS_DEV_URL;#devtool-adding-a-new-recipe-to-the-workspace'><filename>devtool add</filename></ulink>