Add detail to ChangeLog for SLOTH fix

ChangeLog

diff --git a/ChangeLog b/ChangeLog
index 76cd298..558cf9e 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
    * Fix the redefinition of macro ssl_set_bio to an undefined symbol