Add LICENSE.md

LICENSE.md

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+<!--
+SPDX-FileCopyrightText: 2021 Johannes Loher
+
+SPDX-License-Identifier: MIT
+-->
+
+This project is being developed under the terms of the
+[LIMITED LICENSE AGREEMENT FOR MODULE DEVELOPMENT] for Foundry Virtual Tabletop.
+
+The project itself is licensed under multiple licenses. It is difficult to keep
+this section up to date, so here is a brief summary as of June 2021:
+* The images in [img] are licensed under [CC BY 4.0].
+* The files in [.yarn] by the [Yarn] Contributors are licensed under
+  [BSD-2-Clause].
+* Other files are licensed under [MIT].
+
+For more accurate information, check the individual files.
+
+[Yarn]: https://github.com/yarnpkg/berry
+
+[LIMITED LICENSE AGREEMENT FOR MODULE DEVELOPMENT]: https://foundryvtt.com/article/license/
+[CC BY 4.0]: LICENSES/CC-BY-4.0.txt
+[MIT]: LICENSES/MIT.txt
+[BSD-2-Clause]: LICENSES/BSD-2-Clause.txt
+
+[img]: img
+[.yarn]: .yarn