Merge pull request #1012 from NatashaTheRobot/master

[Swift] Add Readme code snippet with example usage of an ASDisplayNode.

Author: appleguy

README.md

@@ -57,6 +57,22 @@ dispatch_async(_backgroundQueue, ^{
 });
 ```
 
+In Swift:
+
+```swift
+dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0) {
+  let node = ASTextNode()
+  node.attributedString = NSAttributedString(string: "hello")
+  node.measure(CGSize(width: screenWidth, height: CGFloat.max))
+  node.frame = CGRect(origin: CGPointZero, size: node.calculatedSize)
+            
+  // self.view isn't a node, so we can only use it on the main thread
+  dispatch_async(dispatch_get_main_queue()) {
+   self.view.addSubview(node.view)
+  }
+}
+```
+
 AsyncDisplayKit at a glance:
 
 * `ASImageNode` and `ASTextNode` are drop-in replacements for UIImageView and