[docs] Add documentation for `vr-mode`
[docs] Add documentation for `vr-mode`

Add 'vr-mode' state to Scene documentation.
ngokevin/aframe,chenzlabs/aframe,aframevr/aframe,aframevr/aframe,chenzlabs/aframe,ngokevin/aframe,dmarcos/aframe,dmarcos/aframe