Mention code of conduct in readme

Author: marijnh

README.md

@@ -20,3 +20,8 @@ There's a [forum](http://discuss.prosemirror.net) for general
 discussion and support requests, and the
 [Github bug tracker](https://github.com/prosemirror/prosemirror/issues)
 is the place to report issues.
+
+We aim to be an inclusive, welcoming community. To make that explicit,
+we have a [code of
+conduct](http://contributor-covenant.org/version/1/1/0/) that applies
+to communication around the project.