All I'm trying to do is do what I would normally have done in full Visual Studio -- put code in a child project and reference that project from a parent project.
Install Visual Studio Community Edition for Mac from the Xamarin home page.
Create a Xamarin Forms app and call it MyApp. Choose Android and iOS targets. That gets you a solution with the following projects in it:
MyApp MyApp.Droid MyApp.iOS
Right click the solution and say "Add New Project". Choose Multiplatorm -> Portable Library (or anything else). Call it "bob".
See bob in the solution?
How do you add a reference to bob from MyApp?
While we are on this, how do we add a NuGet package reference to MyApp? You can add one to MyApp.Droid, MyApp.iOS and bob (try it -- right click on each project, Add -> Add NuGet Packages
Darn'dest thing isn't it?
NOTE: My first guess was that it's a limitation of the free stuff. Well, that doesn't seem to hold up because you can add references between all the other projects. For all of that to be useful in my case, I need to be able to add a reference to the main one (the one with the App.xaml in it)