Document passing arbitrary options to serializer

See discussion in 
- https://github.com/rails-api/active_model_serializers/issues/599#issuecomment-144904554

But is basically the example by @jszwedko

PostController

``` ruby
...
  def show 
    render json: @post, user_id: 1234
  end
...
```

PostSerializer

``` ruby
...
  def my_comments
    Comments.where(user_id: instance_options[:user_id], post_id: object.id)
  end
...
```
