AUTHORS.md: New

Author: Matthias Koeppe

AUTHORS.md

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+# Authors of SageMath #
+
+[Over 800 people](https://www.sagemath.org/development-map.html) have
+contributed code to the Sage library (see [src/](src/)) or have
+contributed to the maintenance of the SageMath distribution (see
+[build/](build/)).
+
+See https://www.sagemath.org/development-map.html for information
+about the developers and their contributions.