Update repositories.txt

Arduino library for fast decibel routines.

Author: RobTillaart

repositories.txt

@@ -5473,6 +5473,7 @@ https://github.com/RobTillaart/DAC8571
 https://github.com/RobTillaart/DAC8574
 https://github.com/RobTillaart/DAC53001
 https://github.com/RobTillaart/dateTimeHelpers
+https://github.com/RobTillaart/decibel
 https://github.com/RobTillaart/DEVFULL
 https://github.com/RobTillaart/DEVNULL
 https://github.com/RobTillaart/DEVRANDOM