Add template code of conduct

[davidlmobley]

Perhaps we want to provide a draft template code of conduct for how people should be using the relevant GitHub repositories (commenting, etc.), see livecomsjournal/livecomsjournal.github.io#21 .

[jchodera]

@orbeckst's group has done a wonderful job of putting together a nice Code of Conduct for MDAnalysis here. It borrow heavily form several great community codes of conduct. This could be a good model for the journal as well.