Update README.md

in the README is explained that you can use the :using parameter, but it wont work as described. I had a really hard time trying to figure it out. After a lot of try and error, finally I figured it out. In README says like this:

```ruby
has_scope :by_period, :using => [:started_at, :ended_at]
```

but it will not work unless you specify the :type parameter, so it must be: 

```ruby
has_scope :by_period, :using => [:started_at, :ended_at], :type => :hash
``` 

then you need to pass the parameters like this:

    graduations/?by_period[started_at]=20120701&by_period[ended_at]=20131013

finally, in the controller you should do:

```ruby
scope :by_period, -> started_at, ended_at { where("started_at = ? AND ended_at = ?", started_at, ended_at) }
```

This way I think It will help others that could have the same problem as I did to figure it out.

(tested on rails 4)