Note regarding CSRF-Token added to POST request rest api docs. (#4613)

frappe/docs/user/en/guides/integration/rest_api.md

@@ -210,6 +210,8 @@ _Response:_
 		"parentfield": null
 	  }
 	}
+	
+Note: `POST` requests are to be sent along with `X-Frappe-CSRF-Token:<csrf-token>` header.
 
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