Setup automatic Slack Invitations

[remarcable]

Motivation

Currently https://uikit-cross-platform.slack.com/ leads to the slack, but users can't join.

Proposed solution

Setup something like https://upscri.be/ (as GraphQL Apollo does) or https://github.com/nacyot/slack_invitation

[janek]

Good catch, should be fixed. I think replacing the link in the README with http://bit.ly/uikit-cross-platform should solve this, right?

[remarcable]

lgtm!