Update repositories.txt (#3276)

Experimental library for elementary math of mixing water with different temperatures.

Author: RobTillaart

repositories.txt

@@ -1765,6 +1765,7 @@ https://github.com/RobTillaart/Temperature
 https://github.com/RobTillaart/Troolean
 https://github.com/RobTillaart/UUID
 https://github.com/RobTillaart/VolumeConverter
+https://github.com/RobTillaart/WaterMix
 https://github.com/RobTillaart/WaveMix
 https://github.com/RobTillaart/X9C10X
 https://github.com/RobTillaart/XMLWriter