Try using reuse to manage copyright and license notes

.gitignore

@@ -1,3 +1,5 @@
+# SPDX-FileCopyrightText: 2016 Contributors to the P4 Project
+#
 # SPDX-License-Identifier: Apache-2.0
 
 # Python byte code

CONTRIBUTING.md

@@ -1,5 +1,9 @@
+<!--
+SPDX-FileCopyrightText: 2024 Contributors to the P4 Project
+
+SPDX-License-Identifier: Apache-2.0
+-->
 
-[comment]: # (SPDX-License-Identifier:  Apache-2.0)
 ---
 
 Thank you for your interest in contributing to the P4 Compiler tutorials repository! Your contributions are important and will help to improve the project for everyone. Before you begin, please consider the guidelines below.