Add grant by MongoDB, Inc.

Received by Github comment:
https://github.com/zeromq/libzmq/pull/1451#issuecomment-537169373

RELICENSE/mongodb.md

+# Permission to Relicense under MPLv2 or any other OSI approved license chosen by the current ZeroMQ BDFL
+
+This is a statement by Jonathan Reams (of MongoDB, Inc.)
+that grants permission to relicense its 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 "jbreams", with
+commit author "Jonathan Reams jbreams@mongodb.com" (specifically the commits merged in pull requests #1451, #1449, #1448, and #1174), are copyright of
+MongoDB, Inc.
+This document hereby grants the libzmq project team to relicense libzmq,
+including all past, present and future contributions of the author listed
+above.
+
+Jonathan Reams
+2019/10/1