policies: add Code of Conduct policy

[zimbatm]

The idea is to help set the tone of our work internally and externally. And also hold ourselves to high standards.

[piegamesde]

Text looks pretty good to me, only thing I'd add is a short reference to the general Code of Conduct we do have now.

[zimbatm]

@NixOS/foundation please take a look so we can move forward.

[nixos-discourse]

This pull request has been mentioned on NixOS Discourse. There might be relevant details there:

https://discourse.nixos.org/t/is-there-a-policy-for-removing-foundation-members-or-what-is-considered-acceptable-behavior/43159/2