Slack link in README will stop working soon

[mcupak]

The link for Slack signup in the README will stop working soon. The service is currently running on old Heroku Cedar-14 stack, which becomes deprecated on May 1, 2020. The service will continue to run after that, but we won't be able to build or update it, so it might go down at some point after without recovery. Normal Slack invites (as opposed to self-signups) will continue working. We should probably look into either deploying a new version of the service or updating the info for joining.