updating README to fix 404 error issue mounting using Cascade (#2089)

Co-authored-by: Jonathan Chan <[email protected]>

Author: jonmchan

CHANGELOG.md

@@ -6,6 +6,8 @@
 
 #### Fixes
 
+* Your contribution here.
+* [#2089](https://github.com/ruby-grape/grape/pull/2089): Specify order of mounting Grape with Rack::Cascade in README - [@jonmchan](https://github.com/jonmchan).
 * [#2083](https://github.com/ruby-grape/grape/pull/2083): Set `Cache-Control` header only for streamed responses - [@stanhu](https://github.com/stanhu).
 
 ### 1.4.0 (2020/07/10)

README.md

@@ -350,9 +350,12 @@ class Web < Sinatra::Base
 end
 
 use Rack::Session::Cookie
-run Rack::Cascade.new [API, Web]
+run Rack::Cascade.new [Web, API]
 ```
 
+Note that order of loading apps using `Rack::Cascade` matters. The grape application must be last if you want to raise custom 404 errors from grape (such as `error!('Not Found',404)`). If the grape application is not last and returns 404 or 405 response, [cascade utilizes that as a signal to try the next app](https://www.rubydoc.info/gems/rack/Rack/Cascade). This may lead to undesirable behavior showing the [wrong 404 page from the wrong app](https://github.com/ruby-grape/grape/issues/1515). 
+
+
 ### Rails
 
 Place API files into `app/api`. Rails expects a subdirectory that matches the name of the Ruby module and a file name that matches the name of the class. In our example, the file name location and directory for `Twitter::API` should be `app/api/twitter/api.rb`.