Add change log

Signed-off-by: Ronald Cron <ronald.cron@arm.com>

ChangeLog.d/tls12-2nd-client-hello.txt

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+Security
+   * Fixed an issue in TLS 1.3 server handling of the second ClientHello, after
+     sending a HelloRetryRequest message. A man-in-the-middle attacker could
+     force a TLS 1.3 session resumption using a ticket to fall back to an
+     unintended TLS 1.2 session resumption with an all-zero master secret.
+     This could result in client authentication being bypassed and allow client
+     impersonation.
+     Found and reported by Jaehun Lee, Pohang University of Science and
+     Technology (POSTECH).