Even though currently (Nov 6, 2015) developers can add App Previews to tvOS apps as there is currently only the option to upload screenshots, the videos can be uploaded to other services for showing what the app is capable of doing. Other services which can be used are Youtube, Dailymotion, Vimeo, and Facebook. Also, self-hosting and sending through email for documentation and testing are other ideas.
- Connect Apple TV with your Mac with USB-C cable
- Connect smart tv through HDMI to Apple TV
- Open QuickTime Player on your Mac
- File Menu -> New Movie Recording
- Select Camera Apple TV (v mark, right side of Recording button)
- The Apple TV screen will appear on QuickTime Player
- Push recording button
Anything you do on Apple TV will be recorded.
All this was done using 10.11 El Capitan and Quicktime Player v. 10.4 (854).