README.md in rakuten_web_service-1.2.0 vs README.md in rakuten_web_service-1.3.0

@@ -57,10 +57,12 @@
 
 ### Configuration
 
 At first, you have to specify your application's key. And you can tell the client your afiiliate id with `RakutenWebService.configuration`.
 
+#### In Your Code
+
 ```ruby
   RakutenWebService.configuration do |c|
     # (Required) Appliction ID for your application.
     c.application_id = 'YOUR_APPLICATION_ID'
 
@@ -78,9 +80,14 @@
     c.debug = true # default: false
   end
 ```
 
 Please note that you need to replace `'YOUR_APPLICATION_ID'` and `'YOUR_AFFILIATE_ID'` with actual ones you have.
+
+#### Environment Variables
+
+You can configure `application_id` and `affiliate_id` by defining environment variables `RWS_APPLICATION_ID` and `RWS_AFFILIATION_ID`.
+
 
 ### Search Ichiba Items
 
 ```ruby
   items = RakutenWebService::Ichiba::Item.search(keyword: 'Ruby') # This returns Enumerable object