Fix and add a link in README
Fix and add a link in README

Fix quick start link and add a link for Slack invitations

redcanaryco/atomic-red-team,redcanaryco/atomic-red-team,redcanaryco/atomic-red-team,redcanaryco/atomic-red-team,redcanaryco/atomic-red-team,redcanaryco/atomic-red-team,redcanaryco/atomic-red-team,redcanaryco/atomic-red-team,redcanaryco/atomic-red-team,redcanaryco/atomic-red-team,redcanaryco/atomic-red-team,redcanaryco/atomic-red-team