Added mention of API mode complications to README (#5041) [ci skip]

README.md

@@ -56,6 +56,7 @@ It's composed of 10 modules:
 	- [ActiveJob Integration](#activejob-integration)
 	- [Password reset tokens and Rails logs](#password-reset-tokens-and-rails-logs)
 	- [Other ORMs](#other-orms)
+	- [Rails API mode](#rails-api-mode)
 - [Additional information](#additional-information)
 	- [Heroku](#heroku)
 	- [Warden](#warden)
@@ -694,6 +695,17 @@ config.log_level = :warn
 
 Devise supports ActiveRecord (default) and Mongoid. To select another ORM, simply require it in the initializer file.
 
+### Rails API Mode
+
+Rails 5+ has a built-in [API Mode](https://edgeguides.rubyonrails.org/api_app.html) which optimizes Rails for use as an API (only). One of the side effects is that it changes the order of the middleware stack, and this can cause problems for `Devise::Test::IntegrationHelpers`. This problem usually surfaces as an ```undefined method `[]=' for nil:NilClass``` error when using integration test helpers, such as `#sign_in`. The solution is simply to reorder the middlewares by adding the following to test.rb:
+
+```ruby
+Rails.application.config.middleware.insert_before Warden::Manager, ActionDispatch::Cookies
+Rails.application.config.middleware.insert_before Warden::Manager, ActionDispatch::Session::CookieStore
+```
+
+For a deeper understanding of this, review [this issue](https://github.com/plataformatec/devise/issues/4696).
+
 ## Additional information
 
 ### Heroku