Update index.md

HfT Building Models/Sketchup/Casper/index.md

@@ -4,7 +4,8 @@ All files herein are created for
 
     Sketchup 8 and above
     
-CityGML specific features ( like Location Info, Surface Types, Grouping and Attributes ) are stored in the models but are only visible, editable and usable with
+CityGML surface types are classified in different layers and are visible and usable in native Sketchup, 
+further CityGML specific features ( like Location Info, Surface Types, Grouping and Attributes ) are stored in the models but are only visible, editable and usable with
 
     CityEditor plugin from 3DIS ( see https://www.3dis.de/cityeditor/ )