Added comments regarding tests for dashboard graph

Added comment regarding tests for dashboard graph. This feature should
be migrated to Ecma6 and had its own tests once Consul reaches
compatibility with Rails 5.1.

app/assets/javascripts/dashboard_graphs.js

@@ -2,6 +2,8 @@
 //= require c3
 //= require c3ext
 
+// TODO: This module is complex enough to require its own tests. Rewrite it using Ecma6 class syntax and
+// write tests for this feature after consul has been migrated to Rails 5.1
 (function(){
   'use strict';