add lib (#6594)

* Update repositories.txt

* Update repositories.txt

* Update repositories.txt

Author: GyverLibs

repositories.txt

@@ -2883,6 +2883,7 @@ https://github.com/gvandersel/SXAccessoire
 https://github.com/GypsyRobot/CuteBuzzerSounds
 https://github.com/GypsyRobot/Formulinha
 https://github.com/GypsyRobot/MusicBuzzer
+https://github.com/GyverLibs/StringN
 https://github.com/GyverLibs/RGBLED
 https://github.com/GyverLibs/UART_RF
 https://github.com/GyverLibs/uPID/