bsp-guide: Added writernotes style

(From yocto-docs rev: 25afb2b10fb278b6ae8519c703a7e56f87096668)

Signed-off-by: Kristi Rifenbark <kristi.rifenbark@gmail.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>

documentation/bsp-guide/bsp-style.css

@@ -730,6 +730,9 @@ div.navfooter {
   border-color: black;
 }
 
+.writernotes {
+  color: red;
+}
 
   /*********** /
  /  graphics  /