Add technical details to the Play design doc

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 want to help work on it.

I've submitted some changes to Play to make it easier to implement Gradle support for Play and am happy to be a resource on the Play side for this. I've added details about those changes to the design doc. I've also 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 could be worked on right after that