Add ChangeLog entry for new features.

Fixes #346
diff --git a/ChangeLog b/ChangeLog
index 7b50534..990d7ca 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -2,6 +2,11 @@
 
 = mbed TLS x.x.x branch released xxxx-xx-xx
 
+Features
+   * Add new crypto primitives from RFC 7539: stream cipher Chacha20, one-time
+   authenticator Poly1305 and AEAD construct Chacha20-Poly1305. Contributed by
+   Daniel King (#485).
+
 API Changes
    * Extend the platform module with a util component that contains
      functionality shared by multiple Mbed TLS modules. At this stage