Create a community code of conduct to help new contributors

As per OpenSource Guide and Github's recommendations, we should have a code of conduct defined for our projects.

It's helpful as we expand into new areas such as Comet, Twitch and Maven Central that are likely to draw contributors from outside our existing community. Having a code of conduct makes it easier for new people to know what we expect from them and helps people know what to expect from us.

Rather than creating one file per repo as per Github's suggestions (we have quite a lot of them now), it'd be better to put documents like this in a central, easily accessible place. We can generate a web page from this repo so it's accessible to everyone/linkable on the channel/etc...

Feedback is open to anyone with an account on the site, so questions and comments welcome.