Accessing a Renderer from Xamarin.Forms

Question

I need to access the Renderer Base Classes as mentioned at the following:

https://learn.microsoft.com/en-us/xamarin/xamarin-forms/app-fundamentals/custom-renderer/renderers

This says they are located at the following namespace:

Xamarin.Forms.Platform.Android

However, this namespace does not seem to be available in the shared project of Xamarin.Forms. I'm not sure what I need to add to my shared project to make this available, can anybody help me? Thanks.