It says here! at the bottom of the page....
Xcode 7 supports 3D Touch development. All the debugging features of Xcode are available for implementing the new features.
Keep the following in mind:
With Xcode 7.0 you must develop on a device that supports 3D Touch. Simulator in Xcode 7.0 does not support 3D Touch.
With Xcode 7.0 you must implement your peek and pop view controllers in code. Interface Builder in Xcode 7.0 does not provide graphical support for configuring view controllers or transitions for 3D Touch.
Be sure to test your app with 3D Touch both enabled and disabled, ensuring that all features are available to all users. On a 3D Touch device, you can disable 3D Touch in Settings > General > Accessibility > 3D Touch
So unfortunately it looks like we need to get our hands on the actual devices to do it. :-(
As of Xcode 7.3 there is a way to do this if your MacBook's trackpad supports force touch. See answer by @iPrabu.