Add a CODE_OF_CONDUCT.md

[oliverklee]

I'd like to use the [Contributor Covenant Code of Conduct"(http://contributor-covenant.org/version/1/4/) for this, which more or less is the standard CoC for projects on GitHub that have a CoC.

Part of the document needs to be an email address to which people can write when they experience/witness harassment or other violations of the CoC. Which email address should I use for this?

[oliverklee]

Fixed with #49 in 0b1089d.