Adding license header

Author: shweaver-MSFT

Microsoft.Toolkit.Uwp/Helpers/ObjectStorage/ApplicationDataStorageHelper.CacheFolder.cs

@@ -1,3 +1,7 @@
+// Licensed to the .NET Foundation under one or more agreements.
+// The .NET Foundation licenses this file to you under the MIT license.
+// See the LICENSE file in the project root for more information.
+
 using System;
 using System.Collections.Generic;
 using System.Threading.Tasks;