Document extra_css and extra_js template blocks usage

EXTENDING.md

@@ -169,3 +169,23 @@ Optionaly, you can customize which URLs are plugged and to which prefix via the
         class MyAppConfig(AppConfig, coin.apps.AppURLS):
             name = 'my_app'
             exported_urlpatterns = [('coolapp', 'my_app.cool_urls')]
+
+Of course, you can add as many additional views as you want.
+
+Templates
+---------
+
+app-specific templates and static files should be placed according to
+[the reusable apps layout](https://docs.djangoproject.com/en/1.9/intro/reusable-apps/#your-project-and-your-reusable-app).
+
+- E.g. app-specific css : *<app folder>/static/<app name>/css/local.css*
+- E.g. app-specific template : *<app folder>/templates/<app name>/base.html*
+
+In order to load app-specific *CSS* and *JavaScript*, you may want to use the
+*extra_css* and *extra_js* template blocks, defined in main *base.html*.
+
+Example:
+
+    {% extends "base.html" %}
+    {% block extra_css %}<link rel="stylesheet" href="{% static "myapp/css/local.css" %}">{% endblock %}
+    {% block extra_js %}<script>alert("So extra !");</script>{% endblock %}