Added a comment giving some instructions on how to get internal method visible to this assembly

WpfViewModelFirst.Tests/MainWindowViewModelTests.cs

@@ -70,6 +70,10 @@ namespace WpfViewModelFirst.Tests
             Assert.IsType<Part2ViewModel>(main.CurrentViewModel);
             Assert.Single(main.Strings);
         }
+        // This method requires the Part1Action method to be internal not private
+        // The MainWindowViewModel class also needs to have an attribute allowing
+        // this assembly to see the internals
+        // [assembly:System.Runtime.CompilerServices.InternalsVisibleTo("WpfViewModelFirst.Tests")]
         [Fact]
         public async Task Part1ActionWillSetCurrentViewModel()
         {