[2.2 migration] Add a link to Akka 2.2 migration guide

Since Play 2.2 upgraded Akka to 2.2, which is not backwards compatible, it would be nice to add a note with a link to their migration guide: http://doc.akka.io/docs/akka/2.2.0/project/migration-guide-2.1.x-2.2.x.html