added a cookbook about managing assets

doc/cookbook/assets.rst

+Managing Assets in Templates
+============================
+
+A Silex application is not always hosted at the web root directory. To avoid
+repeating the base path whenever you link to another page, it is highly
+recommended to use the :doc:`URL generator service provider
+</providers/url_generator>`.
+
+But what about images, stylesheets, or JavaScript files? Their URLs are not
+managed by the Silex router, but nonetheless, they need to get prefixed by the
+base path. Fortunately, the Request object contain the application base path
+that needs to be prepended::
+
+    // generate a link to the stylesheets in /css/styles.css
+    $request->getBasePath().'/css/styles.css';
+
+And doing the same in a Twig template is as easy as it can get:
+
+.. code-block:: jinja
+
+    {{ app.request.basepath }}/css/styles.css
+
+If your assets are hosted under a different host, you might want to abstract
+the path by defining a Silex parameter::
+
+    $app['asset_path'] = 'http://assets.examples.com';
+
+Using it in a template is as easy as before:
+
+.. code-block:: jinja
+
+    {{ app.asset_path }}/css/styles.css
+
+If you need to implement some logic independently of the asset, define a
+service instead::
+
+    $app['asset_path'] = $app->share(function () {
+        // implement whatever logic you need to determine the asset path
+
+        return 'http://assets.examples.com';
+    });
+
+Usage is exactly the same as before:
+
+.. code-block:: jinja
+
+    {{ app.asset_path }}/css/styles.css
+
+If the asset location depends on the asset type or path, you will need more
+abstraction; here is one way to do that with a Twig function::
+
+    $app['twig'] = $app->share($app->extend('twig', function($twig, $app) {
+        $twig->addFunction(new \Twig_SimpleFunction('asset', function ($asset) {
+            // implement whatever logic you need to determine the asset path
+
+            return sprintf('http://assets.examples.com/%s', ltrim($asset, '/'));
+        }));
+
+        return $twig;
+    }));
+
+The ``asset`` function can then be used in your templates:
+
+.. code-block:: jinja
+
+    {{ asset('/css/styles.css') }}

doc/cookbook/index.rst

@@ -15,6 +15,7 @@ The cookbook section contains recipes for solving specific problems.
     sub_requests
     error_handler
     multiple_loggers
+    assets
 
 Recipes
 -------
@@ -38,3 +39,5 @@ Recipes
 * :doc:`Converting Errors to Exceptions <error_handler>`.
 
 * :doc:`Using multiple Monolog Loggers <multiple_loggers>`.
+
+* :doc:`Managing Assets in Templates <assets>`.