Add temperature units

The unit of the values returned by `getTemperature()` were not listed.  I confirmed empirically that the sensor returns the temperature in units of Celsius.

Author: rnowling

docs/api.md

@@ -149,7 +149,7 @@ Represents a Modulino Thermo module.
   Returns the humidity reading.
 
 - **`float getTemperature()`**  
-  Returns the temperature reading.
+  Returns the temperature reading in Celsius.
 
 ---
 
@@ -195,4 +195,4 @@ Represents a color for Modulino Pixels modules.
 
 ## License
 
-This library is released under the [MPL-2.0 license](../LICENSE).
+This library is released under the [MPL-2.0 license](../LICENSE).