Support out-of-tree testing with CMake: add ChangeLog entry.

Fixes #1193

ChangeLog

diff --git a/ChangeLog b/ChangeLog
index 8db0215..5d86733 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -1,5 +1,10 @@
 mbed TLS ChangeLog (Sorted per branch, date)
 
+= mbed TLS 2.x.x branch released xxxx-xx-xx
+
+Changes
+   * Support TLS testing in out-of-source builds using cmake. Fixes #1193.
+
 = mbed TLS 2.7.0 branch released 2018-02-03
 
 Security