docs(README.md): identify as example project and deprecate VRTK.Prefabs
docs(README.md): identify as example project and deprecate VRTK.Prefabs

Highlight this as just an example project and mention the fact that
VRTK.Prefabs has now been deprecated in place of the Tilia repos.
thestonefox/SteamVR_Unity_Toolkit,gomez-addams/SteamVR_Unity_Toolkit,thestonefox/VRTK