Docs:  Can't use group_by(&:purchased_at) in chart method

In the README...
```ruby 
  # Define a query that works on another query's result set
  # Note: chart and data are aliases for query
  chart :deliveries_over_time, through: :deliveries do |results|
    results.group_by(&:purchased_at)
  end
```

The `results` given to the chart block is an ActiveRecord::HashWithIndifferentAccess and not an ActiveRecord result. 
