Adding a Code of Conduct to MacPorts

[Michael Dickens]

Related, sort of: < https://www.newyorker.com/science/elements/after-years-of-abusive-e-mails-the-creator-of-linux-steps-aside >

In my experience, we in MacPorts-land don't have the obvious Linux issue. I hope others don't experience the MacPorts project (developers, users, commenters) as abusive in any medium (email lists, tickets / issues, PR's, in person, whatever). - MLD