Add detail to ChangeLog for SLOTH fix
diff --git a/ChangeLog b/ChangeLog
index 3fd8e0e..2489ce4 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -5,6 +5,7 @@
 Security
    * Removed MD5 from the allowed hash algorithms for CertificateRequest and
      CertificateVerify messages, to prevent SLOTH attacks against TLS 1.2.
+     Introduced by interoperability fix for #513.
 
 Bugfix
    * Fixed potential arithmetic overflow in mbedtls_ctr_drbg_reseed() that could