a comment added to explain the use of HashMap

to avoid misconception of contributors about using HashMap to implement
a JSON object as a unordered collection by the definition

JSONObject.java

@@ -164,6 +164,7 @@ public class JSONObject {
      * Construct an empty JSONObject.
      */
     public JSONObject() {
+        // HashMap is used on purpose to ensure that elements are unordered
         this.map = new HashMap<String, Object>();
     }