message: added doc for Principled and CarPaint
diff: [imgDiffuseRooms]: diffuse_rooms.png { width=80% }
  [imgNormalMap]: normalmap_frustum.png { width=60% }
  [imgMaterialOBJ]: material_OBJ.jpg { width=60% }
  +[imgMaterialPrincipled]: material_Principled.jpg { width=60% }
  +[imgMaterialCarPaint]: material_CarPaint.jpg { width=60% }
  [imgMaterialMetal]: material_Metal.jpg { width=60% }
  [imgMaterialAlloy]: material_Alloy.jpg { width=60% }
  [imgMaterialGlass]: material_Glass.jpg { width=60% }
  