Clarified the code structure for Rails.

I ran into an issue running a very simple Grape API within a Rails 4 application that has to do with the directory structure in Rails. 

It wasn't clear to me in http://intridea.github.io/grape/docs/index.html#Rails, that underneath the `app/api` directory, you need to create yet another subdirectory which name matches the name of the module that implements the Grape API.

For example, if your API is implemented in a file called `api.rb` as:

```ruby
module Twitter
  class API < Grape::API
    version 'v1', using: :path
    format :json

    resource :statuses do
      desc "Return a public timeline."
      get :public_timeline do
        Status.limit(20)
      end
    end
  end
end
```
The file `api.rb` needs to live in `app/api/twitter/api.rb`, otherwise, Rails does not recognize the name of the module and throws the following error when trying to run the rails server: 

`/config/routes.rb:[line number]:in `block in <top (required)>': uninitialized constant Twitter (NameError)`