Add 'Contributors to project' as example

Co-Authored-By: Matija Šuklje <matija@suklje.name>

Signed-off-by: Carmen Bianca BAKKER <carmenbianca@fsfe.org>

Author: carmenbianca

spec.md

@@ -269,6 +269,7 @@ Examples of valid Copyright Notices:
 ```
 SPDX-FileCopyrightText: 2019 Jane Doe <jane@example.com>
 SPDX-FileCopyrightText: © 2019 John Doe <joe@example.com>
+SPDX-FileCopyrightText: Contributors to Example Project <https://project.example.com>
 SPDX-SnippetCopyrightText: (C) Example Cooperative <info@coop.example.com>
 © Example Corporation <https://corp.example.com>
 Copyright 2016, 2018-2019 Joe Anybody