Add Changelog entry for RSA exponent blinding

ChangeLog

diff --git a/ChangeLog b/ChangeLog
index 7117d40..9cc5683 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -2,6 +2,13 @@
 
 = mbed TLS 2.x.x branch released xxxx-xx-xx
 
+Security
+   * Add exponent blinding to RSA private operations as a countermeasure
+     against side-channel attacks like the cache attack described in
+     https://arxiv.org/abs/1702.08719v2.
+     Found and fix proposed by Michael Schwarz, Samuel Weiser, Daniel Gruss,
+     Clémentine Maurice and Stefan Mangard.
+
 Feature
    * Add a new configuration option to 'mbedtls_ssl_config' to enable
      suppressing the CA list in Certificate Request messages. The default