Add a code of conduct.
Add a code of conduct.

Lots of people won't contribute to a project without one. We want the
maximum range of contributors possible.

I've added myself and @sgrif as maintainers for contact should an event
occur that requires mantainer intervention. Should this list be
bigger/smaller?

tohou/diesel,sgrif/diesel,SoftwareWarlock/diesel,SoftwareWarlock/diesel,tohou/diesel,mcasper/diesel,cascalheira/diesel,diesel-rs/diesel,sgrif/diesel,cascalheira/diesel,mcasper/diesel,diesel-rs/diesel