Add entry for HF Slack in communications.md
We have officially settled on Slack as our informal, asynchronous and unofficial chat application. There are new points of difference decided on a recent call between myself, Michael, and Tom:
Anyone is allowed in. We'll have to determine a moderation policy as a result, but we feel an open forum is better in this case.
We will announce this officially, and provide an open invite link, which will be posted on social media and be given its own entry on the HF website.
The existing rules written in
communications.mdare already sound with respect these changes, and need only a small update with the relevant info.