Add ChangeLog entry for clang version fix. Issue #1072

ChangeLog

diff --git a/ChangeLog b/ChangeLog
index 0646e6c..c02dc84 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -24,6 +24,9 @@
 
 Changes
    * Changed CMake defaults for IAR to treat all compiler warnings as errors.
+   * Changed the Clang parameters used in the CMake build files to work for
+     versions later than 3.6. Versions of Clang earlier than this may no longer
+     work. Fixes #1072
 
 = mbed TLS 2.10.0 branch released 2018-06-06