Add instructions for non-scalar attributes

I have a model with a attribute that I choose to serialize in an array. `strong_parameters` disallows non-scalar by default. They do, however, allow them if you pass in an empty array as detailed in their [instructions](https://github.com/rails/strong_parameters#permitted-scalar-values):

![rails_strong_parameters](https://f.cloud.github.com/assets/385448/1132927/1c96c35a-1bd4-11e3-9918-097f4ad96f32.jpg)

In this library (permitters), options are [extracted out of the arguments](https://github.com/permitters/permitters/commit/46d4ed6b3685513aee56d849001478f3e022903d#L28R30), so just providing the argument like this:

```ruby
permit :favorite_colors => []
```

will not add it to the list of permitted parameters. So... a second, empty options hash is necessary to get around it.