pk_sign() now requires non-NONE md_alg for ECDSA

ChangeLog

diff --git a/ChangeLog b/ChangeLog
index e8f63d1..4b6d21f 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -6,6 +6,7 @@
   * Support for DTLS 1.0 and 1.2 (RFC 6347).
 
 API Changes
+   * pk_sign() no longer accepts md_alg == POLARSSL_MD_NONE with ECDSA.
    * Last argument of x509_crt_check_key_usage() changed from int to unsigned.
    * test_ca_list (from certs.h) is renamed to test_cas_pem and is only
      available if POLARSSL_PEM_PARSE_C is defined (it never worked without).