From Miguel de Icaza:
We use a library profile that is better suited for mobile devices, so we removed features that are not necessary (like the entir
ITNOA
Maybe PCLAppConfig is help you to create and read from app.config in Xamarin.Forms PCL Project or other Xamarin projects.
app.config
For different configuration in different build mode such as release and debug you can use Configuration Transform on app.config.