Middleware in guide updated [ci skip]

guides/source/rails_on_rack.md

@@ -131,6 +131,7 @@ use ActionDispatch::DebugExceptions
 use ActionDispatch::RemoteIp
 use ActionDispatch::Reloader
 use ActionDispatch::Callbacks
+use ActiveRecord::Migration::CheckPending
 use ActiveRecord::ConnectionAdapters::ConnectionManagement
 use ActiveRecord::QueryCache
 use ActionDispatch::Cookies
@@ -272,6 +273,11 @@ Much of Action Controller's functionality is implemented as Middlewares. The fol
 
 * Runs the prepare callbacks before serving the request.
 
+
+ **`ActiveRecord::Migration::CheckPending`**
+
+* Checks pending migrations and raise an error if migrations are pending.
+
  **`ActiveRecord::ConnectionAdapters::ConnectionManagement`**
 
 * Cleans active connections after each request, unless the `rack.test` key in the request environment is set to `true`.