Seeing as I pretty much did this last night (albeit with a different sensor), I thought I might as well put it on here for others anyway
Create an automation as follows, and everytime the state of the sensor (the value in this case) changes, it will trigger and publish to the MQTT topic:
You’ll have to change the entity_id for the trigger, to that of the Darksky humidity sensor, then set the topic in the action to whatever topic you wish to publish to. Lastly, change the payload_template to match the Darksky humidity sensor.
I’m using the above to push the humidity reading from one Home Assistant install, to another via MQTT. That way I have have a “live” system and a development system using the same sensor names and values.