Update repositories.txt (#3450)

Added URL to a new library that we will use for an Arduino-based high-altitude weather balloon.

Author: F-Schmidt99

repositories.txt

@@ -6288,3 +6288,4 @@ https://github.com/ito-soft-design/irboard_esp32
 https://github.com/m5stack/M5Cardputer
 https://github.com/arduino-libraries/Arduino_ScienceKitCarrier
 https://github.com/adafruit/Adafruit_VCNL4020
+https://gitlab.com/F-Schmidt/ulm_weatherballoon