Add cad link

3D/beepberry-cases/slim_flat/README.md

@@ -2,6 +2,8 @@
 <img src=images/01.jpg width=200> <img src=images/02.jpg width=200>  
 <img src=images/03.jpg width=200> <img src=images/04.jpg width=200>  
   
+You can modify or export this design in different formats here: https://cad.onshape.com/documents/bd480b14f09d0e8bb1f02c35/w/8fd70d525d2eaf5fd9ab36e5/e/2a70bcd406a8aed3ac3e3c04?renderMode=0&uiState=6482bcb4e9963043625c78c9.  
+  
 **Parts List**
 * Two printed parts
 * Two plastic screws - I used these: https://www.amazon.com/gp/product/B00GDYNJNM.  Maybe you can find silber buttonhead that match the m2.5 screws.