Added copyright guidelines

Signed-off-by: Rob Tjalma <[email protected]>

Author: Rob Tjalma

CONTRIBUTING.md

@@ -15,10 +15,12 @@ The following is a set of guidelines for contributing to the CoMPAS project. The
   * [Contributing Code](#contributing-code)
   * [Tools to contribute](#tools-to-contribute)
   * [Definition of Done](#definition-of-done)
+  * [Copyright Guidelines](#definition-of-done)
 
 [Styleguides](#styleguides)
   * [Git Commit Messages](#git-commit-messages)
   * [Java Styleguide](#java-styleguide)
+  * [Copyright Guidelines](#copyright-guidelines)
 
 [Project Governance](#project-governance)
   * [Project Owner](#project-owner)
@@ -180,6 +182,23 @@ As a simple yet instructive example, consider ...
  */
 ```
 
+### Copyright Guidelines
+
+Contributing to the CoMPAS project also requires to use correct copyright headers in source files.
+
+For each CoMPAS repository, we created / will create a Github Action featuring [REUSE](https://reuse.software/). REUSE is a piece of software which checks for correct copyright information in files defined in a [specification](https://reuse.software/spec/). The specification is based on best practices and the use of [SPDX](https://spdx.dev/) identifiers.
+
+Example Alliander copyright header for Java files:
+```Java
+// SPDX-FileCopyrightText: 2020 Alliander N.V.
+//
+// SPDX-License-Identifier: Apache-2.0
+```
+
+Every commit on a Pull Request is being scanned by REUSE. If it fails, the pull requests cannot be merged.
+
+For more tips on using REUSE (for example with a small command line tool), check the [Tips: Copyright & Licensing](https://wiki.lfenergy.org/pages/viewpage.action?pageId=10996220) wiki page.
+
 ### English language convention
 
 The convention for all the project's documents, including code documentation, website, is to write American English.