Add Slack setup to success criteria for ITP Welcome week (#1769)

Per issue #1431: Add Slack setup to success criteria so trainees know they need to join class channels

Author: chinar-amrutkar

org-cyf-itp/content/welcome/success/index.md

@@ -9,6 +9,7 @@ objectives = [[
   "Name 5 people in your class.",
   "Have a plan for how to make progress, and how to get help.",
   "Find and begin your prep for the first sprint of the Onboarding Module.",
+  "Set up Slack and join your class channels.",
 ]]
 +++