Add message about internal inclustion of IsExternalInit

UnitTests/UnitTests.UWP/IsExternalInit.cs

@@ -4,6 +4,13 @@
 
 namespace System.Runtime.CompilerServices
 {
+    // HACK (2021.05.07): Included as a workaround for multiple occurrences of 
+    // error CS0518: Predefined type 'System.Runtime.CompilerServices.IsExternalInit' is not defined or imported
+    // in UnitTests\UnitTests.UWP\UI\Controls\Test_RangeSelector.cs.
+    // This is caused by using [Positional Records](https://docs.microsoft.com/en-us/dotnet/csharp/whats-new/csharp-9#record-types) 
+    // which use [Init Only setters](https://docs.microsoft.com/en-us/dotnet/csharp/whats-new/csharp-9#init-only-setters) under the hood for all the properties,
+    // and currently the type is only included net5.0 and up.
+    // The recommended action is to include the type manually. https://developercommunity.visualstudio.com/t/error-cs0518-predefined-type-systemruntimecompiler/1244809#T-N1249582
     internal static class IsExternalInit
     {
     }