A few weeks ago, I exclusively revealed that Windows RT 8.1 Update 3 would introduce the Start Menu to Windows RT devices, and with it bring a few other minor additions. Today, Microsoft has confirmed that the Start Menu is indeed coming in Update 3, along with improvements to the lockscreen.
Microsoft has updated it's Windows 10 FAQ (via WMPU) page with more details regarding the update for Windows RT users:
If you’re running Windows RT, your device won’t upgrade to Windows 10. We will have an update available in September 2015 that will improve the Start menu and lock screen. Check Windows Update on your Windows RT device to make sure it is ready to download the update when available.