Update README.md

CAD/3dp-cell-holder/README.md

@@ -3,4 +3,5 @@ This 3D printed part allows to replace the four small parts holding the test sam
 # Advangtages of this design
 - fewer parts which makes the assembly easier
 - the cell is better hold which prevent variation in results due to change in the orientation of the cell
-- a side extention prevents the UV LED to directly light the sensor
+- a side extention wall prevents the UV LED to directly light the sensor
+- a bottom wall prevents light from comming from the bottom of the cell trough gaps - more consistency in test conditions