clarify introductory component example (#526)

* Update index.md

I tried to follow the guide and every example worked, except for the components. Upon further investigation, I found the issue: the component must be registered before the Vue instance. This was not clear in the Introductory guide, so I suggest it's added ( I copied the same sentence as seen on https://vuejs.org/guide/components.html )

* clarify introductory component example

Author: emanuelgaspar

src/guide/index.md

@@ -296,6 +296,10 @@ Now we can pass the todo into each repeated component using `v-bind`:
 </div>
 ```
 ``` js
+Vue.component('todo-item', {
+  props: ['todo'],
+  template: '<li>{{ todo.text }}</li>'
+})
 var app7 = new Vue({
   el: '#app-7',
   data: {