Add RELICENSE entry

Signed-off-by: Rosen Penev <rosenp@gmail.com>

RELICENSE/neheb.md

+# Permission to Relicense under MPLv2
+
+This is a statement by Rosen Penev
+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 "flub", with
+commit author "Rosen Penev <rosenp@gmail.com>", are copyright of
+Rosen Penev .
+This document hereby grants the libzmq project team to relicense libzmq,
+including all past, present and future contributions of the author listed above.
+
+Rosen Penev
+2019/04/19