Now you will add the ability to load files to your application. Once again, NSDocument has taken care of most of the details for you.
To do the unarchiving, you will use NSKeyedUnarchiver, which has the following handy method:
class func unarchiveObjectWithData(data: NSData) -> AnyObject?
In Document.swift, edit your readFromData(_:ofType:error:) method to look like this:
override func readFromData(data: NSData, ofType typeName: String, error outError: ...