Includes

## Change
Add a DSL method to set the relations that should be eager loaded

## Before
```ruby
ActiveAdmin.register Post do
  controller do
    def scoped_collection
      super.includes :author, :categories
    end
  end
end
```

## After
```ruby
ActiveAdmin.register Post do
  includes :author, :categories
end
```
we should add a other example for overwriting `scoped_collection`