Add relicense agreement from Spotify AB

RELICENSE/spotify.md

+# Permission to Relicense under MPLv2
+
+This is a statement by Spotify AB 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 "gimaker", with commit author
+"Staffan Gimåker", the commits made by the Github handle "danielnorberg", with
+commit author "Daniel Norberg", and the commits made by the Github handle
+"caipre", with commit author "Nick Platt", are copyright of Spotify AB. This
+document hereby grants the libzmq project team to relicense libzmq, including
+all past, present and future contributions of the authors listed above.
+
+Nick Platt <nickplatt@spotify.com>
+2019/08/21