Add a contributing file

CONTRIBUTING.rst

@@ -0,0 +1,14 @@
+Contributing to the Slixmpp project
+===================================
+
+To contribute, the preferred way is to commit your changes on some
+publicly-available git repository (on a fork `on github
+<https://github.com/poezio/slixmpp>`_ or on your own repository) and to
+notify the developers with either:
+ - a ticket `on the bug tracker <https://dev.poez.io/new>`_
+ - a pull request on github
+ - a simple message on `the XMPP MUC <xmpp:poezio@muc.poez.io>`_
+
+Even though Slixmpp’s github repository is just a read-only mirror, we can
+still be notified of the pull requests and fetch your mirror manually to
+integrate your changes.