Add Code of Conduct to cfengine/core and cfengine/masterfiles

We lack a Code of Conduct. While we do manage behavior, it's not formalized, and that's requested when looking to join other initiatives like https://fixme.ossn.club/

Resources:

• https://github.com/mendersoftware/mender/blob/master/code-of-conduct.md
• https://www.contributor-covenant.org/version/1/0/0/code-of-conduct/
• https://opensource.guide/code-of-conduct/
• https://www.chef.io/code-of-conduct/chef-contributor-covenant/
• https://docs.ansible.com/ansible/latest/community/code_of_conduct.html
• https://puppet.com/community/community-guidelines/