Update readme to point to newer Si7021 humidity sensor lib

Author: nseidle

README.md

@@ -18,10 +18,11 @@ Repository Contents
 Documentation
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 * **[MPL3115A2 Library](https://github.com/sparkfun/SparkFun_MPL3115A2_Breakout_Arduino_Library)** - MPL3115A2 library required for Weather Shield examples. 
-* **[HTU21D Library](https://github.com/sparkfun/SparkFun_HTU21D_Breakout_Arduino_Library)** - HTU21D library required for Weather Shield examples. 
+* **[Si7021 Library](https://github.com/sparkfun/SparkFun_Si701_Breakout_Arduino_Library)** - Si7021 humidity sensor library required for Weather Shield examples. 
 * **[Hookup Guide](https://learn.sparkfun.com/tutorials/weather-shield-hookup-guide)** - Basic hookup guide for the Weather Shield.
 * **[SparkFun Fritzing repo](https://github.com/sparkfun/Fritzing_Parts)** - Fritzing diagrams for SparkFun products.
 * **[SparkFun 3D Model repo](https://github.com/sparkfun/3D_Models)** - 3D models of SparkFun products. 
+* **[HTU21D Library](https://github.com/sparkfun/SparkFun_HTU21D_Breakout_Arduino_Library)** - The HTU21D humidity sensor was used on older versions of the Weather Shield so if you have an older version you may need this library to run the examples.
 
 License Information
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