Added note on Adafruit_Sensor requirement.

Author: microbuilder

README.md

@@ -5,6 +5,9 @@ Designed specifically to work with the Adafruit BME280 Breakout
 
 These sensors use I2C or SPI to communicate, up to 4 pins are required to interface
 
+Use of this library also required [Adafruit_Sensor](https://github.com/adafruit/Adafruit_Sensor)
+to be installed on your local system.
+
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