Update README with Swift example

I was able to get this working in Swift by setting the window property in AppDelegate.swift as such:

``` swift
import SmudgeKit

@UIApplicationMain
class AppDelegate: UIResponder, UIApplicationDelegate {
  var window: UIWindow? = SmudgyWindow(frame: UIScreen.mainScreen().bounds)
  //[...]
}
```

The README mentions some bug in the swift compiler. Works fine in Swift 2.2 it seems.
