Make the files compatible to reuse specification

[maxhbr]

For easier incorporation in open source projects, especially if they try to be reuse compliant, it would be great if the formats that support comments would contain license information.

The reuse specification recommends for that in markdown the following simple comment:

<!--
SPDX-FileCopyrightText: 2014 Coraline Ada Ehmke
SPDX-FileCopyrightText: ...
SPDX-FileCopyrightText: ...

SPDX-License-Identifier: CC-BY-4.0
-->

ping @mxmehl