Problem: missing relicensing statement from mgatny

Solution: add it

RELICENSE/mgatny.md

+# Permission to Relicense under MPLv2 or any other OSI approved license chosen by the current ZeroMQ BDFL
+
+This is a statement by Mike Gatny (mgatny) 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 "mgatny", with commit author
+"Mike Gatny <mgatny@gmail.com>" or "Mike Gatny <mgatny@connamara.com>", are
+copyright of Mike Gatny.  This document hereby grants the libzmq project team
+to relicense libzmq, including all past, present and future contributions of
+the author listed above.
+
+Mike Gatny
+2019/02/19