Added ebean to migration guide

There are currently two things that are Play specific about it:

* The plugin automatically adds a dependency on `play-ebean` to your project, which is Play specific (provides Play modules etc).  This is more of a convenience than anything else.
* The plugin depends on the Play SBT plugin.  The reason for this is to ensure ordering in settings loaded, because the Play SBT plugin depends on the Play enhancer, and the ebean byte code enhancement must run after the play getters/setters are generated.  Once [this PR](https://github.com/sbt/sbt/pull/1714) is merged on SBT and eventually released, we'll have more flexibility here, since byte code enhancers will be a configurable list - we'll probably push the responsibility onto the user to explicitly add the byte code enhancers they want to run in what order in their build configuration.  Currently this functionality is implemented by overriding the compile task, which makes for these strange ordering requirements.