Code of Conduct

I've added a basic code of conduct (see #5), opening this PR to start some discussion around it.

One thing that became apparent is that we'd probably need a second version for organisers which explains how to deal with complaints etc. [The template here](http://geekfeminism.wikia.com/wiki/Conference_anti-harassment/Policy#Internal_version_for_conference_staff) would be a good starting point, from the same place as the code of conduct.