Add invite_user command to CLI help text (#3229)

mattermost.go

@@ -1254,6 +1254,11 @@ COMMANDS:
         Example:
             platform -create_user -team_name="name" -email="user@example.com" -password="mypassword" -username="user"
 
+    -invite_user                      Invites a user to a team by email. It requires the -team_name
+                                        and -email flags.
+        Example:
+            platform -invite_user -team_name="name" -email="user@example.com"
+
     -join_team                        Joins a user to the team.  It required the -email and
                                        -team_name.  You may need to logout of your current session
                                        for the new team to be applied.