Update README.md

Author: hiro08gh

README.md

@@ -32,7 +32,7 @@ import { createClient } from 'microcms-js-sdk'; //ES6
 
 // Initialize Client SDK.
 const client = createClient({
-  serviceDomain: "YOUR_DOMAIN",
+  serviceDomain: "YOUR_DOMAIN", // YOUR_DOMAIN is the XXXX part of XXXX.microcms.io
   apiKey: "YOUR_API_KEY",
   globalDraftKey: "YOUR_GLOBAL_DRAFT_KEY", // If need 
 });