Updates ChangeLog with faster MPI zeroize fix

Added optimised mbedtls_mpi_zeroise() credit to ChangeLog.
diff --git a/ChangeLog b/ChangeLog
index bee652c..15bb337 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -26,7 +26,8 @@
    * On ARM platforms, when compiling with -O0 with GCC, Clang or armcc5,
      don't use the optimized assembly for bignum multiplication. This removes
      the need to pass -fomit-frame-pointer to avoid a build error with -O0.
-   * Disabled SSLv3 in the default configuration. 
+   * Disabled SSLv3 in the default configuration.
+   * Optimized mbedtls_zeroize() for MPI integer size. (Fix by Alexey Skalozub)
 
 = mbed TLS 2.2.1 released 2016-01-05