docs(README): shorten what's in the box section
docs(README): shorten what's in the box section

The What's In The Box section was a bit too long and all
of the text didn't really belong in the general README.
This additional information can move to the docs site.
gomez-addams/SteamVR_Unity_Toolkit,gpvigano/SteamVR_Unity_Toolkit,gpvigano/VRTK,thestonefox/VRTK,mattboy64/SteamVR_Unity_Toolkit,Fulby/VRTK,red-horizon/VRTK,virror/VRTK,thestonefox/SteamVR_Unity_Toolkit,adamjweaver/VRTK,Innoactive/IA-unity-VR-toolkit-VRTK,TMaloteaux/SteamVR_Unity_Toolkit