Wrap the copyright and licence header in HTML comment tags.

CONTRIBUTING.md

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+<!--
  * Copyright (C) 2020 Eclipse Foundation and others. 
  * 
  * This program and the accompanying materials are made available under the
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  * SPDX-FileType: DOCUMENTATION
  * SPDX-FileCopyrightText: 2020 Eclipse Foundation
  * SPDX-License-Identifier: EPL-2.0
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+-->
 
 # Contributing to Eclipse Dash, Tools for Committers