Update repositories.txt (#4597)

Arduino library for PCR process control.

Author: RobTillaart

repositories.txt

@@ -5174,6 +5174,7 @@ https://github.com/RobTillaart/PCA9685_RT
 https://github.com/RobTillaart/PCF8574
 https://github.com/RobTillaart/PCF8575
 https://github.com/RobTillaart/PCF8591
+https://github.com/RobTillaart/PCR
 https://github.com/RobTillaart/PERIPUMP
 https://github.com/RobTillaart/PID_RT
 https://github.com/RobTillaart/PinInGroup