Add links to new example 20, shading.

graph2d_examples.html

@@ -227,6 +227,12 @@
             <div class="exampleTitle">labels</div>
         </a>
     </div>
+    <div class="col-lg-2 col-md-3 col-sm-6">
+        <a href="examples/graph2d/20_shading.html">
+            <img class="example img-responsive" src="./images/exampleScreenshots/graph2d/20.png">
+            <div class="exampleTitle">shading</div>
+        </a>
+    </div>
 </div>
 <br />
 <br />
@@ -238,4 +244,4 @@
 <!-- Include all compiled plugins (below), or include individual files as needed -->
 <script src="js/bootstrap.min.js"></script>
 </body>
-</html>
+</html>