Link to custom fields demo from demos/index.html

demos/custom-fields/index.html

@@ -18,6 +18,11 @@
       is used to define a turtle.
     </p>
 
+    <p>All of the custom field implementation is in
+     demos/custom-fields/field_turtle.js, including comments on each required
+     function.
+    </p>
+
     <p>Click on the blocks' comment icons to learn what they are demonstrating.
       Use the buttons below to see how the fields react to changes.
     </p>

demos/index.html

@@ -149,6 +149,18 @@
       </td>
     </tr>
 
+    <tr>
+      <td>
+        <a href="custom-fields/index.html">
+          <img src="custom-fields/icon.png" height=80 width=100>
+        </a>
+      </td>
+      <td>
+        <div><a href="custom-fields/index.html">Custom Fields</a></div>
+        <div>Implement a custom field.</div>
+      </td>
+    </tr>
+
     <tr>
       <td>
         <a href="storage/index.html">