Add NOTICE.txt and update license headers.

I am not a lawyer, but to the best of my knowledge this is what I'm supposed to do according to Apache 2.0, section 4

Author: natemcmaster

Directory.Build.props

@@ -1,7 +1,7 @@
 <Project>
 
   <PropertyGroup>
-    <Authors>Microsoft;Nate McMaster</Authors>
+    <Authors>Nate McMaster</Authors>
     <Product>CommandLineUtils</Product>
     <Copyright>Copyright © Nate McMaster</Copyright>
     <NeutralLanguage>en-US</NeutralLanguage>

LICENSE.txt

@@ -1,14 +1,13 @@
-Copyright (c) .NET Foundation and Contributors
+Copyright Nate McMaster
 
-All rights reserved.
-
-Licensed under the Apache License, Version 2.0 (the "License"); you may not use
-this file except in compliance with the License. You may obtain a copy of the
-License at
+Licensed under the Apache License, Version 2.0 (the "License");
+you may not use this file except in compliance with the License.
+You may obtain a copy of the License at
 
     http://www.apache.org/licenses/LICENSE-2.0
 
-Unless required by applicable law or agreed to in writing, software distributed
-under the License is distributed on an "AS IS" BASIS, WITHOUT WARRANTIES OR
-CONDITIONS OF ANY KIND, either express or implied. See the License for the
-specific language governing permissions and limitations under the License.
+Unless required by applicable law or agreed to in writing, software
+distributed under the License is distributed on an "AS IS" BASIS,
+WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+See the License for the specific language governing permissions and
+limitations under the License.

NOTICE.txt

@@ -0,0 +1,21 @@
+This project is a fork of https://github.com/aspnet/Common and https://github.com/aspnet/DotNetTools.
+The modified work is available under the license found in LICENSE.txt in the root of this project.
+Any files that have been modified include a notice stating that they have been modified.
+
+The unmodified work was originally licensed under the following terms:
+----------------------------------------------------------------------------------------------------
+
+Copyright (c) .NET Foundation and Contributors
+
+All rights reserved.
+
+Licensed under the Apache License, Version 2.0 (the "License"); you may not use
+this file except in compliance with the License. You may obtain a copy of the
+License at
+
+    http://www.apache.org/licenses/LICENSE-2.0
+
+Unless required by applicable law or agreed to in writing, software distributed
+under the License is distributed on an "AS IS" BASIS, WITHOUT WARRANTIES OR
+CONDITIONS OF ANY KIND, either express or implied. See the License for the
+specific language governing permissions and limitations under the License.

samples/HelloWorld/Program.cs

@@ -1,4 +1,7 @@
-using System;
+// Copyright (c) Nate McMaster.
+// Licensed under the Apache License, Version 2.0. See License.txt in the project root for license information.
+
+using System;
 using McMaster.Extensions.CommandLineUtils;
 
 public class Program

src/CommandLineUtils/CommandArgument.cs

@@ -1,5 +1,6 @@
-// Copyright (c) .NET Foundation. All rights reserved.
+// Copyright (c) Nate McMaster.
 // Licensed under the Apache License, Version 2.0. See License.txt in the project root for license information.
+// This file has been modified from the original form. See Notice.txt in the project root for more information.
 
 using System.Collections.Generic;
 using System.Linq;

src/CommandLineUtils/CommandLineApplication.cs

@@ -1,5 +1,6 @@
-// Copyright (c) .NET Foundation. All rights reserved.
+// Copyright (c) Nate McMaster.
 // Licensed under the Apache License, Version 2.0. See License.txt in the project root for license information.
+// This file has been modified from the original form. See Notice.txt in the project root for more information.
 
 using System;
 using System.Collections;

src/CommandLineUtils/CommandLineApplicationExtensions.cs

@@ -1,5 +1,6 @@
-// Copyright (c) .NET Foundation. All rights reserved.
+// Copyright (c) Nate McMaster.
 // Licensed under the Apache License, Version 2.0. See License.txt in the project root for license information.
+// This file has been modified from the original form. See Notice.txt in the project root for more information.
 
 using System;
 using System.Reflection;

src/CommandLineUtils/CommandOption.cs

@@ -1,5 +1,6 @@
-// Copyright (c) .NET Foundation. All rights reserved.
+// Copyright (c) Nate McMaster.
 // Licensed under the Apache License, Version 2.0. See License.txt in the project root for license information.
+// This file has been modified from the original form. See Notice.txt in the project root for more information.
 
 using System;
 using System.Collections.Generic;

src/CommandLineUtils/CommandOptionType.cs

@@ -1,5 +1,6 @@
-// Copyright (c) .NET Foundation. All rights reserved.
+// Copyright (c) Nate McMaster.
 // Licensed under the Apache License, Version 2.0. See License.txt in the project root for license information.
+// This file has been modified from the original form. See Notice.txt in the project root for more information.
 
 namespace McMaster.Extensions.CommandLineUtils
 {

src/CommandLineUtils/CommandParsingException.cs

@@ -1,5 +1,6 @@
-// Copyright (c) .NET Foundation. All rights reserved.
+// Copyright (c) Nate McMaster.
 // Licensed under the Apache License, Version 2.0. See License.txt in the project root for license information.
+// This file has been modified from the original form. See Notice.txt in the project root for more information.
 
 using System;