add config.ru example file

and require sinatra/base

asset_management/sprockets.md

@@ -127,7 +127,7 @@ Summing up, your main application file may look like this:
 # app/app.rb
 
 # install and require all dependencies
-require 'sinatra'
+require 'sinatra/base'
 require 'sprockets'
 require 'uglifier'
 require 'sass'
@@ -158,6 +158,14 @@ class MyApp < Sinatra::Base
 end
 ```
 
+```ruby
+# app/config.ru
+
+require './app'
+
+run MyApp.new
+```
+
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