Added documentation for new device class 'plug' (#3889)

* Added documentation for new device class 'plug'

See home-assistant/frontend#550

* use term "unplugged"

* add the entry to the right place

source/_components/binary_sensor.markdown

@@ -24,6 +24,7 @@ The way these sensors are displayed in the frontend can be modified in the [cust
 - **moving**: `On` means moving, `Off` means stopped
 - **occupancy**: `On` means occupied, `Off` means not occupied
 - **opening**: `On` means open, `Off` means closed
+- **plug**: `On` means device is plugged in, `Off` means device is unplugged
 - **power**: Power, over-current, etc.
 - **safety**: `On` means unsafe, `Off` means safe
 - **smoke**: `On` means smoke detected