An initial, I have done it post, with more detail to follow. Hopefully this is new and not redoing something else that has been done. But I was interested in getting generic zigbee sensors into Indigo that wouldnt sync via the Hue hub and the great Hue plugin (which I use for all my bulbs). I also didnt want to buy another hub.
Using a £10 USB stick (cheaper if you wait for Ali shipping) I have got a £10 temp/humidity/pressure sensor that looks great updating 3 linked devices in Indigo via MQTT. No other hardware required. This is opening up a new range of sensor options.