Add precipitation device class (#1517)

Author: epenet

docs/core/entity/number.md

@@ -54,6 +54,7 @@ If specifying a device class, your number entity will need to also return the co
 | pm10 | µg/m³ | Concentration of particulate matter less than 10 micrometers |
 | power | W, kW | Power.
 | power_factor | % | Power Factor
+| precipitation | in, mm | Precipitation
 | precipitation_intensity | in/d, in/h, mm/d, mm/h | Precipitation intensity
 | pressure | cbar, bar, hPa, inHg, kPa, mbar, Pa, psi | Pressure.
 | reactive_power | var | Reactive power |

docs/core/entity/sensor.md

@@ -55,6 +55,7 @@ If specifying a device class, your sensor entity will need to also return the co
 | pm10 | µg/m³ | Concentration of particulate matter less than 10 micrometers |
 | power | W, kW | Power, statistics will be stored in W.
 | power_factor | % | Power Factor
+| precipitation | in, mm | Precipitation
 | precipitation_intensity | in/d, in/h, mm/d, mm/h | Precipitation intensity
 | pressure | cbar, bar, hPa, inHg, kPa, mbar, Pa, psi | Pressure, statistics will be stored in Pa.
 | reactive_power | var | Reactive power |