Describe how to read optional fields / objects with Json Transformers

[gmethvin]

(Moved from playframework/playframework#6756)

Actually reading optional fields / objects with Json Transformers is possible, but it is currently missing in our documentation, so we should add something like that to our docs: http://stackoverflow.com/a/15998881/2250209

Actually the important part is: .orElse(__.json.pick[JsObject])