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

[NatashaTheRobot]

This makes it easier to glance at for those who plan to use the project in Swift.

[appleguy]

@NatashaTheRobot hey, thank you! I've seen you on twitter (before earlier today of course, too) - great to have you here :). I'm happy to say that another community member, @lappp9, is starting a huge push on updating the website and the documentation for 2.0 (note the 1.9.x releases contain most of the features that 2.0 will, but has not GM'd due to the lack of documentation).

Our strategy regarding Swift, briefly, is as follows:

• We're now actively listening to the Swift community for feedback on the API design and what improvements can be made; some of our top contributors are are using Swift in their day jobs.
• The new, redesigned example app that the 2.0 programming guide will walk through will be written with both Objective-C and Swift (although, the initial version will start with Objective-C).
• The site will include an inline switcher for each code snippet to toggle between the languages.
• In 6-12 months, we'll likely be Swift-first for example apps and documentation changes, with back-ports to Objective-C continuing for a couple years. Usage within the framework will probably start around this time, especially if 3.0 means the runtime won't need to be bundled in each app.

Considering the extreme lack of documentation for rather powerful APIs like ASLayoutSpec, don't hesitate to reach out with any feedback or questions — I'd sincerely enjoy talking to you about it! I can give you my iMessage number if you email asyncdisplaykit@gmail.com.