Add ChangeLog entry for config-no-entropy.h change

ChangeLog

diff --git a/ChangeLog b/ChangeLog
index da9ee0b..c349a66 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -22,6 +22,8 @@
    * Removed mutexes from ECP hardware accelerator code. Now all hardware
      accelerator code in the library leaves concurrency handling to the
      platform. Reported by Steven Cooreman. #863
+   * Define the macro MBEDTLS_AES_ROM_TABLES in the configuration file
+     config-no-entropy.h to reduce the RAM footprint.
 
 = mbed TLS 2.5.1 released 2017-06-21