ref-manual: Added style "writernotes" to *.css stylesheet.

This addition allows an author to use the following form to
highlight a paragraph in red.

   <para role="writernotes">
   Blah Blah Blah
   </para>

(From yocto-docs rev: 40d646229e5ec9fa905a372be795120449f9f9dd)

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

documentation/ref-manual/ref-style.css

@@ -773,6 +773,10 @@ div.navfooter {
   border-color: black;
 }
 
+.writernotes {
+  color: red;
+}
+
 
   /*********** /
  /  graphics  /