we need a contributing.md for ISC DHCP

We had some stuff about how to contribute on the web site, I think that content is mostly now here: https://kb.isc.org/docs/contributing-to-iscs-open-source

it can be boilerplate. it should probably cover at least:

• how to submit a patch (presumably now via gitlab?)
• that your contribution will be covered by the current license for the project
• that you need to submit tests and doc for your contribution
• if there is still a contrib directory, what are the rules for what goes in there (e.g. ldap stuff?)