Add RAIN - Arduino library for a rain-sensor (#2188)

Author: RobTillaart

repositories.txt

@@ -3573,6 +3573,7 @@ https://github.com/RobTillaart/PrintSize
 https://github.com/RobTillaart/PrintString
 https://github.com/RobTillaart/PulsePattern
 https://github.com/RobTillaart/RADAR
+https://github.com/RobTillaart/RAIN
 https://github.com/RobTillaart/randomHelpers
 https://github.com/RobTillaart/rotaryDecoder
 https://github.com/RobTillaart/rotaryDecoderSwitch