They are not officially supported, but many people have made them work. However, you have to be careful with how you build your Zigbee network: they do not work well with most repeaters and will "fall off" your network if they route through one.
You also need custom drivers, but that's the easy part. 
The rest Is summarized here: Xiaomi & Aqara Devices - Pairing & Keeping them connected. If you are totally new to Hubitat and Zigbee, I'm not sure I'd recommend them, but most people can get them to work.
