Create credits.md

Somehow, the binary package missed this critical file. Oops.

Author: ChallengeSY

credits.md

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+## Nebunoid credits
+Main engine + sound effects + foreground assets by Paul Ruediger. Licensed under [MIT](license.md).
+
+### Included nebulae backgrounds
+* Crab Neubla courtesy of NASA, ESA, and the Arizona State Universe. Public domain.
+* Flame Nebula courtesy of NASA. Public domain.
+* Horsehead Nebula courtesy of NASA, NOAO, ESA, Hubble, and STScl. Public domain.
+* Lagoon Nebula courtesy of ESO. Licensed under Creative Commons (BY 4.0).
+* Omega Nebula courtesy of NASA, ESA, and the Arizona State Universe. Public domain.
+* Orion Nebula courtesy of ESO. Licensed under Creative Commons (BY 4.0).
+* Tarantula Nebula courtesy of NASA and STScl. Public domain.