[dns-resolver-tf] Mattermost instance for chat

Hello, Mirjam and the RIPE NCC have managed to get a Mattermost instance that we can use for chat set up. She'll be sending you an invitation soon. No "official" communication will happen there, but every form of communication has it's own qualitative difference. E-mail is different from voice communication, video conferencing is different from face-to-face meeting, and so on. We should be able to more quickly work out issues in real time if we have a real time forum. All proposed text will eventually get sent to the mailing list though. Cheers, -- Shane