WHATSNEW: Add release notes for Samba 4.8.4.

WHATSNEW: Add release notes for Samba 4.8.4.

o  CVE-2018-1139  (Weak authentication protocol allowed.)
o  CVE-2018-1140  (Denial of Service Attack on DNS and LDAP server.)
o  CVE-2018-10858 (Insufficient input validation on client directory
                   listing in libsmbclient.)
o  CVE-2018-10918 (Denial of Service Attack on AD DC DRSUAPI server.)
o  CVE-2018-10919 (Confidential attribute disclosure from the AD LDAP
                   server.)

Signed-off-by: Karolin Seeger <kseeger@samba.org>