Updating documentation for ThreadPools for Akka 2.2

Akka 2.2 has some updates that require changes to your configuration if you are using the 'highly synchronous' configuration pattern that Play recommends. 

I've updated the documentation for how to configure for 'highly synchronous'  per the akka migration guide: http://doc.akka.io/docs/akka/2.2-M1/project/migration-guide-2.1.x-2.2.x.html