Little improvement in the doc

[Tom Lobato]

Readme:
"If you set a backgroundImage, don't forget to set the Menu View Controller's background color to clear color."

Similarly, window.backgroundColor must have no color set.

in AppDelegate:

        window = UIWindow(frame: UIScreen.mainScreen().bounds)
        if let window = window {
            // do not: window.backgroundColor = UIColor.whitecolor()