Add grant by nixman

Submitted as Github comment:

https://github.com/zeromq/libzmq/pull/242#issuecomment-537903446

RELICENSE/nixman.md

+# Permission to Relicense under MPLv2 or any other OSI approved license chosen by the current ZeroMQ BDFL
+
+This is a statement by niXman (github.nixman@pm.me)
+that grants permission to relicense its copyrights in the libzmq C++
+library (ZeroMQ) under the Mozilla Public License v2 (MPLv2) or any other 
+Open Source Initiative approved license chosen by the current ZeroMQ 
+BDFL (Benevolent Dictator for Life).
+
+A portion of the commits made by the Github handle "niXman", with
+commit author "niXman", are copyright of nixMan .
+This document hereby grants the libzmq project team to relicense libzmq, 
+including all past, present and future contributions of the author listed above.
+
+niXman (github.nixman@pm.me)
+2019/10/03