feat(slack): more specific hello message request

Author: themightychris

event-handlers/Emergence/Slack/team_join/general-welcome.php

@@ -3,6 +3,6 @@
 Emergence\Slack\API::request('chat.postMessage', [
     'post' => [
         'channel' => Emergence\Slack\API::getChannelId('general'),
-        'text' => ":waving: Welcome, <@{$_EVENT['user']['id']}>! What are you hoping to get out of your time with Code for Philly?"
+        'text' => ":waving: Welcome, <@{$_EVENT['user']['id']}>! When you get the chance, please reply to this message in a thread to introduce yourself and mention anything you're hoping to get out of your time with Code for Philly"
     ]
-]);
+]);