Added the comment why es6-map is needed.

redboltz · redboltz · commit 71a178da8949 · 2018-08-23T19:36:59.000+09:00
diff --git a/lib/store.js b/lib/store.js
@@ -11,6 +11,16 @@ var defaultStoreOptions = {
   clean: true
 }
 
+/**
+ * es6-map can preserve insertion order even if ES version is older.
+ *
+ * https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Map#Description
+ * It should be noted that a Map which is a map of an object, especially
+ * a dictionary of dictionaries, will only map to the object's insertion
+ * order. In ES2015 this is ordered for objects but for older versions of
+ * ES, this may be random and not ordered.
+ *
+ */
 var Map = require('es6-map')
 
 /**
