add a bit of guidance with upgrading

Author: ericbmerritt

UPGRADING3.1.md

@@ -0,0 +1,21 @@
+Upgrading From 2.1 to 3.1
+=========================
+
+See the general upgrading guidelines [here](http://www.zeromq.org/docs:3-1-upgrade).
+
+Things to watchout for in erlzmq2
+---------------------------------
+
+The 'timeout' flag has been removed from the send receive flags. In
+3.1 the native zeromq `sndtimeo` and `rcvtimeo` flags where added. You
+should use these instead.
+
+Also, is in zeromq as a whole the hwm flag has been replaced with
+sndhwm and rcvhwm.
+
+Things not to worry about
+-------------------------
+
+The rest of the things in that upgrade guide can be ignored. The
+erlzmq2 nif binding abstracts that away for you.
+