[copyright] add MIT License copyright header to zmq_sub.py

contrib/zmq/zmq_sub.py

@@ -1,4 +1,7 @@
 #!/usr/bin/env python2
+# Copyright (c) 2014-2016 The Bitcoin Core developers
+# Distributed under the MIT software license, see the accompanying
+# file COPYING or http://www.opensource.org/licenses/mit-license.php.
 
 import array
 import binascii