Fix issue 394 and update info

src/Compatibility/IsExternalInit.cs

@@ -0,0 +1,14 @@
+// Compatibility shim for init-only setters in .NET Framework 4.6.2
+// This type is required for C# 9+ init accessors to work in older frameworks
+// See: https://github.com/dotnet/runtime/issues/45510
+
+namespace System.Runtime.CompilerServices
+{
+    /// <summary>
+    /// Reserved for use by the compiler for tracking metadata.
+    /// This class allows the use of init-only setters in .NET Framework 4.6.2.
+    /// </summary>
+    internal static class IsExternalInit
+    {
+    }
+}