add an entry about AP page and action extraction in the upgrading gui…

…de [ci skip]

guides/source/upgrading_ruby_on_rails.md

@@ -69,6 +69,13 @@ in the `config/initializers/wrap_parameters.rb` file:
 
 ### Action Pack
 
+Rails 4.0 has deprecated `ActionController::Base.page_cache_extension` option. Use
+`ActionController::Base.default_static_extension` instead.
+
+Rails 4.0 has removed Action and Page caching from ActionPack. You will need to
+add the `actionpack-action_caching` gem in order to use `caches_action` and
+the `actionpack-page_caching` to use `caches_pages` in your controllers.
+
 Rails 4.0 changed how `assert_generates`, `assert_recognizes`, and `assert_routing` work. Now all these assertions raise `Assertion` instead of `ActionController::RoutingError`.
 
 Rails 4.0 also changed the way unicode character routes are drawn. Now you can draw unicode character routes directly. If you already draw such routes, you must change them, for example: