Implement ISC Code of Conduct for Kea

We have decided/been informed, that we need to extend the ISC code of conduct to Kea. We are going to use the CoC already adopted for the BIND 9 project.

• post Coc in this repo in a separate file (!688 (merged))
• link to coc in readme (!688 (merged))
• email user-list, kea-dev to explain/notify
• add reference to coc on kea-users list signup page (at ISC and Nabble) (waiting for merge to main branch so we can link to the Coc in the repo)
• update bounceback for new subscribers (waiting for merge to main branch so we can link to the Coc in the repo)
• update Reporting Guidelines on ISC web site
• add DHCP Dev Director to reporting team