Made the usage of repositories more clear.

Today I struggled a few hours with setting up the `Lotus::Repository` correctly. It took me quite a while till I recognized that the repository has to be in the same namespace as the model.

```ruby
module Lotus
  module Model
    module Mapping
      class Collection
        def configure_repository!

        # This one here made my initial setup failing...
        repository = Object.const_get("#{ entity.name }#{ REPOSITORY_SUFFIX }")
        repository.collection = name
        rescue NameError
      end
    end
  end
end
```

If it is intended that the repository must have the same namespace it should be documented.