Create code of conduct

[eriktrom]

We don't have a code of conduct, free speech welcome, but derogatory comments towards individuals (not ideas) will not be tolerated.

• Make a code of conduct file, follow community norms (ideally, I like the way rust has handled this, see their's. Copy it or similar).

[Octalbyte]

@eriktrom Is the standard "Contributor Covenant" good enough?