add header

Author: TylerLeonhardt

src/PowerShellEditorServices.Engine/Services/TextDocument/Handlers/CompletionHandler.cs

@@ -1,3 +1,8 @@
+//
+// Copyright (c) Microsoft. All rights reserved.
+// Licensed under the MIT license. See LICENSE file in the project root for full license information.
+//
+
 using System;
 using System.Management.Automation;
 using System.Text.RegularExpressions;