dev-manual: Added a note to the EXTERNALSRC example about the class

Added a short note after the example that shows how to set up
the EXTERNALSRC variables stating that the externalsrc class
needs to be either globally or locally inherited.

(From yocto-docs rev: db935135c0b060e06f6a63530df995d286002598)

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

documentation/dev-manual/dev-manual-common-tasks.xml

@@ -7270,6 +7270,12 @@ Gateways via their Web Interfaces</ulink>"</emphasis>
      EXTERNALSRC = "<replaceable>path</replaceable>"
      EXTERNALSRC_BUILD = "<replaceable>path</replaceable>"
             </literallayout>
+            <note>
+                In order for these settings to take effect, you must globally
+                or locally inherit the
+                <ulink url='&YOCTO_DOCS_REF_URL;#ref-classes-externalsrc'><filename>externalsrc</filename></ulink>
+                class.
+            </note>
         </para>
 
         <para>