Add header comment to default AppDelegate.swift

This is the default for files from Xcode. Even though we at thoughtbot
dislike these header comments, we should use the default behavior here
for Liftoff. This file is easily overridden by a custom app delegate
template, so removing these comments is trivial.

templates/AppDelegate.swift

@@ -1,3 +1,11 @@
+//
+//  AppDelegate.swift
+//  <%= project_name %>
+//
+//  Created by <%= author %> on <%= Time.now.strftime("%-m/%-d/%y") %>
+//  Copyright (c) <%= Time.now.strftime('%Y') %> <%= company %>. All rights reserved.
+//
+
 import UIKit
 
 @UIApplicationMain