Grant by xantares

Set via github comment:

https://github.com/zeromq/libzmq/pull/1366#issuecomment-581451018

RELICENSE/xantares.md

+# Permission to Relicense under MPLv2
+
+This is a statement by Ricardo Catalinas Jimenez
+that grants permission to relicense its copyrights in the libzmq C++
+library (ZeroMQ) under the Mozilla Public License v2 (MPLv2).
+
+A portion of the commits made by the Github handle "xantares", with
+commit author "Michel Zou <xantares09@hotmail.com>", are copyright of Michel Zou.
+This document hereby grants the libzmq project team to relicense
+libzmq,
+including all past, present and future contributions of the author
+listed above.
+
+Michel Zou
+2020/02/03