Add the relicense statement

RELICENSE/benjaminderoche.md

+# Permission to Relicense under MPLv2
+
+This is a statement by Benjamin Deroche
+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 "BenjaminDeroche", with
+commit author "Benjamin Deroche", are copyright of Benjamin Deroche.
+This document hereby grants the libzmq project team to relicense libzmq, 
+including all past, present and future contributions of the author listed above.
+
+Benjamin Deroche
+2019/10/26
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