Updated section 'Configuring controllers'

I added an extra note in the section on creating a custom (namespaced) controller. 

This controller needs to be created in a directory in order for it to work. Otherwise (for example when creating the controller in `app/controllers`) a `uninitialized constant Admins` error is thrown.

Also, fixed the Markdown formatting for the numbered lists in the README.
Merge pull request #2480 from robinroestenburg/patch-1

Updated section 'Configuring controllers'