Add section about the ordering of ModernTheme in App.axaml

It might be a good idea to add a section in the documentation explaining the ordering of styles in App.axaml. 

For example, if you already have an app with a theme and you only want to use some Actipro specific controls you should make sure to add <ModernTheme Includes="..." />  at the top of your styles so that other known control styles will not be override by the modern theme. 

It is general Avalonia behavior but it would make a nice touch to give some guidance in the documentation.