CHANGES: add release note for the permissions

Since the rsync of skeleton or rootfs now changes the permissions of
the copied files and directories, this may break existing setups that
rely on these permissions being kept. So add a release note explaining
how it should be done.

[Thomas: fixup commit log as suggested by Baruch.]

Signed-off-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>