add explanation on 'how to joint'

w3.html

@@ -156,6 +156,8 @@
               <p> If you want to measure the distance between two objects, use 'Measure' tab in 'Inspect'.</p>
               <h5>8. How to joint: Press 'J' key</h5>
               <p> </p>
+              <img src="image/assignment/w3/016.png" width="1110" alt="How to draw a construction line">
+              <img src="image/assignment/w3/017.png" width="1110" alt="How to draw a construction line">
               <h5>9. How to make a hole: Press 'H' key</h5>
               <p> </p>
               <img src="image/assignment/w3/020.png" width="1110" alt="How to draw a construction line">