Create LICENSE.md

Author: aaronolsen

LICENSE.md

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+Shield: [![CC BY-SA 4.0][cc-by-sa-shield]][cc-by-sa]
+
+This work is licensed under a
+[Creative Commons Attribution-ShareAlike 4.0 International License][cc-by-sa].
+
+[![CC BY-SA 4.0][cc-by-sa-image]][cc-by-sa]
+
+[cc-by-sa]: http://creativecommons.org/licenses/by-sa/4.0/
+[cc-by-sa-image]: https://licensebuttons.net/l/by-sa/4.0/88x31.png
+[cc-by-sa-shield]: https://img.shields.io/badge/License-CC%20BY--SA%204.0-lightgrey.svg