CoC note from berlinjs.org

As discussed with @rmehner on Twitter (<https://twitter.com/rmehner/status/519777349370261504>), this adds the same code-of-conduct note as I already added to <http://berlinjs.org>.

It links to the ["Berlin Code of Conduct"](http://rubyberlin.github.io/code-of-conduct/) that contains far more detailed guidelines that just "be excellent to each other".