DOC: bootstrap-select is a dropdown dependency @tejesh.p

docs/dropdown.md

@@ -3,9 +3,14 @@
 `$.dropdown()` creates dropdowns that integrate well with
 [$.urlfilter](#urlfilter) and [$.urlchange](#urlchange).
 
-It requires the [bootstrap-select](https://silviomoreto.github.io/bootstrap-select/examples/)
+It requires the [bootstrap-select](https://developer.snapappointments.com/bootstrap-select/)
 library and its dependencies.
 
+```html
+  <link rel="stylesheet" href="ui/bootstrap-select/dist/css/bootstrap-select.min.css">
+  <script src="ui/bootstrap-select/dist/js/bootstrap-select.min.js"></script>
+```
+
 Example:
 
 ```html