Update UWP guidance to recommend creating the model on a separate project

[divega]

Currently EF Core 2.0 tooling will not work directly with EF Core models in .NET Core. This is described in more detail at dotnet/efcore#9666.

In the short term, we need to update the guidance for working with UWP projects to place the EF Core model in a separate project that is compatible with our tooling.

This could be a .NET Standard library which either cross compiles for .NET Core, is used by a .NET Core console app.