3.0 update own configuration engine

Following to #1672 

Update the XmlConfig example

I was not sure where to promote the addition of the `XmlConfig.php`, I added it in `src/Engine/XmlConfig.php`, 
if you think that `src/Configure/Engine/XmlConfig.php` or any other directory is best suited for that, I'll update the PR.
Name it App\Configure\Engine instead