A message was sent to a channel
{
"type": "message",
"subtype": "group_leave",
"ts": "1358877455.000010",
"user": "U2147483828",
"text": "<@U2147483828|cal> has left the group"
}
A group_leave message is sent when a member of a private group leaves that
group.
When an event occurs, we will send an HTTP POST request to your Request URL. The outer payload is uniform across all Events API methods, and contains fields like team_id and event. Learn more