Added license

Author: ArchieCoder

Microsoft.Toolkit.Uwp.UI.Behaviors/Keyboard/KeyDownTriggerBehavior.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 Microsoft.Xaml.Interactivity;
 using Windows.System;
 using Windows.UI.Xaml;