chore(README): add variant details for headset fading script
chore(README): add variant details for headset fading script

The instructions for using the Headset Collision Fading script are different
for `Unity 5.4` and newer because of the way the `[CameraRig]` prefab is
reshuffled and the `Camera (head)` child object is disabled.

This means the Headset Collision Fading script is required to be added
to the `Camera (eye)` child instead.
Frostlock/SteamVR_Unity_Toolkit,phr00t/SteamVR_Unity_Toolkit,pargee/SteamVR_Unity_Toolkit,bengolds/SteamVR_Unity_Toolkit,adamjweaver/VRTK,Fulby/VRTK,virror/VRTK,nmccarty/htvr,mimminito/SteamVR_Unity_Toolkit,Blueteak/SteamVR_Unity_Toolkit,gpvigano/SteamVR_Unity_Toolkit,jcrombez/SteamVR_Unity_Toolkit,gpvigano/VRTK,TMaloteaux/SteamVR_Unity_Toolkit,gomez-addams/SteamVR_Unity_Toolkit,thestonefox/VRTK,thestonefox/SteamVR_Unity_Toolkit,red-horizon/VRTK,mbbmbbmm/SteamVR_Unity_Toolkit,mattboy64/SteamVR_Unity_Toolkit,Innoactive/IA-unity-VR-toolkit-VRTK,thacio/SteamVR_Unity_Toolkit