Fixes [YOCTO #9963]
I created a new cautionary note to warn the user to use the correct
operator "+=" rather than "=" so that they do not destroy $PN.
(From yocto-docs rev: e6861ffe9400b1ee48cc46ccfc6e8d65efbbec8c)
Signed-off-by: Scott Rifenbark <srifenbark@gmail.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>