Add blog post about sensor temperature conversion

blog/2021-08-12-sensor_temperature_conversion.md

@@ -0,0 +1,32 @@
+---
+author: Erik Montnémery
+authorURL: https://github.com/emontnemery
+title: Temperature conversions moving to SensorEntity
+---
+
+Temperature unit conversions are moving from the `Entity` base class to the `SensorEntity` base
+class. Unit conversions will only be done if the sensor's `device_class` attribute is set
+to `DEVICE_CLASS_TEMPERATURE`. If the `device_class` is not set or is not set to
+`DEVICE_CLASS_TEMPERATURE` temperature conversion will take place during a transition
+period and a warning will be logged.
+
+To facilitate this, the sensor entity model has been updated with two new properties, 
+`native_value` and `native_unit_of_measurement`. This allows us to add additional
+unit conversions in the future instead of relying on the integrations to do it themselves.
+
+Sensor implementations should no longer implement the `state()` property function or set
+the `_attr_state` attribute. Sensor implementations should also not implement the
+`unit_of_measurement` property function, set the `_attr_unit_of_measurement` attribute
+or set the `unit_of_measurement` member of `EntityDescription`.
+
+### native_value
+
+The value reported by the sensor.
+The actual state written to the state machine may be modified by `SensorEntity` due to
+unit conversions.
+
+### native_unit_of_measurement
+
+The unit of measurement of the sensor, if any.
+The `unit_of_measurement` written to the state machine may be modified by `SensorEntity`
+due to unit conversions.