Update WelcomeToMRTK.md
Update WelcomeToMRTK.md

The Unity development journey on docs.microsoft.com currently supports MRTK 2.4 (this will change soon).
killerantz/HoloToolkit-Unity,killerantz/HoloToolkit-Unity,DDReaper/MixedRealityToolkit-Unity,killerantz/HoloToolkit-Unity,killerantz/HoloToolkit-Unity