Add changlog entry.

Signed-off-by: Jerry Yu <jerry.h.yu@arm.com>
diff --git a/ChangeLog.d/workaround_gnutls_anti_replay_fail.txt b/ChangeLog.d/workaround_gnutls_anti_replay_fail.txt
new file mode 100644
index 0000000..fba6f78
--- /dev/null
+++ b/ChangeLog.d/workaround_gnutls_anti_replay_fail.txt
@@ -0,0 +1,6 @@
+Bugfix
+    * Workaround #6623. That is time unit issue. The unit of ticket age is
+      seconds in MBedTLS and milliseconds in GnuTLS. If the real age is 10ms,
+      it might be 1s(1000ms), as a result, the age of MBedTLS is greater than
+      GnuTLS server. Reduce 1 if the age is greater than 1 second to workaround
+      it.