Add docstrings

source/developers/platform_example_sensor.markdown

@@ -31,19 +31,25 @@ from homeassistant.helpers.entity import Entity
 
 
 def setup_platform(hass, config, add_devices, discovery_info=None):
+    """Setup the sensor platform."""
     add_devices([ExampleSensor()])
 
 
 class ExampleSensor(Entity):
+    """Representation of a Sensor."""
+
     @property
     def name(self):
+        """Return the name of the sensor."""
         return 'Example Temperature'
 
     @property
     def state(self):
+        """Return the state of the sensor.""
         return 23
 
     @property
     def unit_of_measurement(self):
+        """Return the unit of measurement."""
         return TEMP_CELSIUS
 ```