Add URL to library.properties

Fixes problem in [this library manager request](arduino/Arduino#8139)

Author: oclyke

library.properties

@@ -5,5 +5,5 @@ maintainer=SparkFun Electronics <sparkfun.com>
 sentence=Library for the SHTC3 humidity and temperature sensor by Sensirion
 paragraph=Library for the SHTC3 humidity and temperature sensor by Sensirion. Read values raw or converted to floating point numbers with units. Enable/disable sleep mode while not being used
 category=Sensors
-url=
-architectures=*
+url=https://github.com/sparkfun/SparkFun_SHTC3_Arduino_Library
+architectures=*