Update Changelog for X.509 unrecognised field fix

ChangeLog

diff --git a/ChangeLog b/ChangeLog
index 87768ff..c242d1f 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -61,6 +61,8 @@
      naming collision in projects which also have files with the common name
      net.c. For consistency, the corresponding header file, net.h, is marked as
      deprecated, and its contents moved to net_sockets.h.
+   * Changed the strategy for X.509 certificate parsing and validation, to no
+     longer disregard certificates with unrecognised fields.
 
 = mbed TLS 2.3.0 branch released 2016-06-28