update readme

README.md

@@ -81,7 +81,8 @@ malformed LAS output for LAS1.4 files with point format 6 to 10.
 
 ## Add points in Las
 
-[add_points_in_las.py](pdaltools/add_points_in_las.py): add points from some vector files (ex: shp, geojson, ...) inside Las. Attributes values are given by default, but could be forced using the 'dimensions' option.  
+[add_points_in_las.py](pdaltools/add_points_in_las.py): add points from some vector files (ex: shp, geojson, ...) inside Las. New points will have X,Y and Z coordinates. Other attributes values given by  the initial las file are null (ex: classification at 0). These others attributes could be forced by using the '--dimensions/-d' option in the command line (ex : 'add_points_in_las.py -i myLas.las -g myPoints.json -d classification=64' - points will have there classification at 64). The dimension should be present in the initial las ; this is not allowed to add new dimension.
+
 
 # Dev / Build