Outdated Slack link in README

[Spacechild1]

The README contains a link to a Slack channel, but I don't think that channel exists anymore. I guess we should just remove it. Maybe we could add a link to the SC Discord server instead?

[telephon]

I may not be the right person to say so, but I think we should direct people as much as possible to scsynth.org – this reduces fragmentation.