On your iOS device, from your lock screen, swipe right to get to the Today view. Or in any app, swipe down from the top of the screen to show your notifications, then swipe right.
At the bottom of the Today view, there's an Edit button. Tap this and look for the Range widget to add. That's it. You'll now see the temperatures of any connected thermometers without having to open the Range app. Tap on a temperature to open the app.