Add permission to relicense

RELICENSE/mkoppanen.md

+# Permission to Relicense under MPLv2 or any other OSI approved license chosen
+# by the current ZeroMQ BDFL
+
+This is a statement by Mikko Koppanen that grants permission to relicense his
+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 "mkoppanen", with commit
+author "Mikko Koppanen <mikko@kuut.io>", are copyright of Mikko Koppanen.
+This document hereby grants the libzmq project team to relicense libzmq,
+including all past, present and future contributions of the author listed
+above.
+
+Mikko Koppanen
+2018/02/17