Add technical details to the Play design doc

@adammurdoch I looked into what would be required to add Play support. I hope these changes to the design doc will be helpful to anyone who might work on it in the future

I added some details about how an incoming Play request triggers a Gradle build. I also, split Milestone 1 into two milestones - the first being the minimum required to get folks with basic project setups using and testing gradle's play support and the second being what should be worked on right after that

One question, what does "Developer runs Scala interactive console" refer to? Is that the sbt console? I.e. the console that is opened when you run the "play" command