Add document about using `extend ActiveModel::Naming`

Hi again, I found a small problem.
I want to separate I18n namespace with model name using inherited Object class.
It is possible by doing it, but it is undocumented.

```ruby
class User
  extend Enumerize
  extend ActiveModel::Naming

  (snip)
end
```

Then I added it to README.md.

Please refactor my English as you like :D