Add signal_strength to device_class of sensor (#212)

* Add signal_strength to device_class of sensor

See home-assistant/architecture#165
Relates to home-assistant/core#22738

* Fix typo

docs/entity_sensor.md

@@ -24,6 +24,7 @@ If specifying a device class, your sensor entity will need to also return the co
 | battery | % | % of battery that is left.
 | humidity | % | % of humidity in the air.
 | illuminance | lx/lm | Light level.
+| signal_strength | dB/dBm | Signal strength.
 | temperature | °C/°F | Temperature.
 | timestamp | ISO8601 | Timestamp.
 | power | W,kW | Power.