Closes #21: Add COPYING

diff --git a/COPYING b/COPYING
new file mode 100644 (file)
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--- /dev/null
+++ b/COPYING
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+This game is distributed under MIT License (see LICENSE).
+
+Textures used in this game:
+Pixel Perfection by XSSheep is licensed under a Creative Commons Attribution-Share Alike 4.0 International License.
+https://creativecommons.org/licenses/by-sa/4.0/