Fixes YOCTO #3662.
Per comments from Paul Eggleton, I updated the description to
include best practices of prepending and from using within
append files.
(From yocto-docs rev: 368055bc046277fe601ff3a64913044be0a15e6d)
Signed-off-by: Scott Rifenbark <scott.m.rifenbark@intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>