Adding missing copyright header.

AsyncRx.NET/Playground/Program.cs

@@ -1,4 +1,8 @@
-using System;
+// Licensed to the .NET Foundation under one or more agreements.
+// The .NET Foundation licenses this file to you under the Apache 2.0 License.
+// See the LICENSE file in the project root for more information. 
+
+using System;
 using System.Linq;
 using System.Reactive.Concurrency;
 using System.Reactive.Linq;