add note

template/src/components/Hello.vue

@@ -8,6 +8,10 @@
 export default {
   data () {
     return {
+      // note: changing this line won't causes changes
+      // with hot-reload because the reloaded component
+      // preserves its current state and we are modifying
+      // its initial state.
       msg: 'Hello World!'
     }
   }