Add CONTRIBUTING.md

Closes zeromq#265

Author: drewnoakes

CONTRIBUTING.md

@@ -0,0 +1,15 @@
+### Contributing to NetMQ
+
+* The contributors are listed in [AUTHORS.md](https://github.com/zeromq/netmq/blob/master/AUTHORS.md) (add yourself).
+
+* This project uses a modified version of the LGPL v3 license, see 
+[COPYING.LESSER](https://github.com/zeromq/netmq/blob/master/COPYING.LESSER).
+
+* We use the [C4.1 (Collective Code Construction Contract)](http://rfc.zeromq.org/spec:22) process for contributions.
+Please read this if you are unfamiliar with it.
+
+* Please maintain the existing code style. 
+
+* Please try to keep your commits small and focussed.
+
+* If the project diverges from your branch, please rebase instead of merging. This makes the commit graph easier to read.