Add units for humidity

I confirmed the humidity units, so add them to the API docs.

Author: rnowling

docs/api.md

@@ -146,7 +146,7 @@ Represents a Modulino Thermo module.
 #### Methods
 
 - **`float getHumidity()`**  
-  Returns the humidity reading.
+  Returns the humidity reading as a percentage in the range 0 to 100.
 
 - **`float getTemperature()`**  
   Returns the temperature reading in Celsius.