Update documentation to customize JavaScript

Note that, while it doesn't really affect the way the application
behaves (as long as the JavaScript code doesn't rely on the order it's
loaded) we're requiring `app/assets/javascripts/custom.js` after
requiring any files in the `app/assets/javascripts/custom/` folder. This
is done for consistency, since we load the content of
`app/assets/javascripts/application.js` after requiring everything else.

app/assets/javascripts/custom.js

@@ -1,7 +1,23 @@
-// Overrides and adds customized javascripts in this file
-// Read more on documentation:
-// * English: https://github.com/consuldemocracy/consuldemocracy/blob/master/CUSTOMIZE_EN.md#javascript
-// * Spanish: https://github.com/consuldemocracy/consuldemocracy/blob/master/CUSTOMIZE_ES.md#javascript
+// Add calls to your custom JavaScript code using this file.
 //
+// We recommend creating your custom JavaScript code under the
+// `app/assets/javascripts/custom/` folder and calling it from here.
 //
+// See the `docs/en/customization/javascript.md` file for more information.
 
+var initialize_modules = function() {
+  "use strict";
+
+  // Add calls to your custom code here; this will be called when
+  // loading a page.
+};
+
+var destroy_non_idempotent_modules = function() {
+  "use strict";
+
+  // Add calls to your custom code here when your JavaScript code added
+  // in `initialize_modules` is not idempotent.
+};
+
+$(document).on("turbolinks:load", initialize_modules);
+$(document).on("turbolinks:before-cache", destroy_non_idempotent_modules);