Update README.md

Link product repo. No need to create new version number since nothing in the library was modified.

Author: bboyho

README.md

@@ -27,6 +27,7 @@ Documentation
 --------------
 * **[Library](https://github.com/sparkfun/Qwiic_Spectral_Sensor_AS726X)** - C library for the AS726X.
 * **[Hookup Guide](https://learn.sparkfun.com/tutorials/as726x-nirvi)** - Basic hookup guide for the AS726X.
+* **[Product Repository](https://github.com/sparkfun/Qwiic_Spectral_Sensor_AS726X)** - Main repository (including hardware files) for the AS726X Spectral Sensor.
 
 Product Versions
 ----------------