clarified exclude usage and RailsEngine Usage

I've added some text in the readme regarding 2 things.

1.  How to dump with primary keys. It looks like the old "WITH_ID" option has been replaced with EXCLUDE . 

2.  You've made changes in the past to support Rails Engines. I added a quick bit about using Rails Engine in the MODELS section.

We are using this gem to sync product data from production back to development. Keeping the primary keys and using Engine's models are needful !