Add a CHANGES entry for CVE-2019-1551

Reviewed-by: NPaul Dale <paul.dale@oracle.com>
(Merged from https://github.com/openssl/openssl/pull/10575)

CHANGES

@@ -9,6 +9,18 @@
 
  Changes between 1.1.1d and 1.1.1e [xx XXX xxxx]
 
+  *) Fixed an an overflow bug in the x64_64 Montgomery squaring procedure
+     used in exponentiation with 512-bit moduli. No EC algorithms are
+     affected. Analysis suggests that attacks against 2-prime RSA1024,
+     3-prime RSA1536, and DSA1024 as a result of this defect would be very
+     difficult to perform and are not believed likely. Attacks against DH512
+     are considered just feasible. However, for an attack the target would
+     have to re-use the DH512 private key, which is not recommended anyway.
+     Also applications directly using the low level API BN_mod_exp may be
+     affected if they use BN_FLG_CONSTTIME.
+     (CVE-2019-1551)
+     [Andy Polyakov]
+
   *) Added a new method to gather entropy on VMS, based on SYS$GET_ENTROPY.
      The presence of this system service is determined at run-time.
      [Richard Levitte]