Restore note about nested SCRIPT_NAME handling.

guides/source/upgrading_ruby_on_rails.md

@@ -145,6 +145,8 @@ Also check your environment settings for `config.action_dispatch.best_standards_
 
 * In Rails 4.0, a generic `ActionDispatch::ParamsParser::ParseError` exception is raised when `ParamsParser` fails to parse request params. You will want to rescue this exception instead of the low-level `MultiJson::DecodeError`, for example.
 
+* In Rails 4.0, `SCRIPT_NAME` is properly nested when engines are mounted on an app that's served from a URL prefix. You no longer have to set `default_url_options[:script_name]` to work around overwritten URL prefixes.
+
 * Rails 4.0 deprecated `ActionController::Integration` in favor of `ActionDispatch::Integration`.
 * Rails 4.0 deprecated `ActionController::IntegrationTest` in favor of `ActionDispatch::IntegrationTest`.
 * Rails 4.0 deprecated `ActionController::PerformanceTest` in favor of `ActionDispatch::PerformanceTest`.