feat(functional): describe how to pass down attributes (vuejs#1405)

* feat(functional): describe how to pass down attributes

* Tweak section about passing attributes to functional components

Author: posva

src/v2/guide/render-function.md

@@ -579,6 +579,24 @@ Vue.component('smart-list', {
 })
 ```
 
+### Passing Attributes and Events to Child Elements/Components
+
+On normal components, attributes not defined as props are automatically added to the root element of the component, replacing or [intelligently merging with](class-and-style.html) any existing attributes of the same name. 
+
+Functional components, however, require you to explicitly define this behavior:
+
+```js
+Vue.component('my-functional-button', {
+  functional: true,
+  render: function (createElement, context) {
+    // Transparently pass any attributes, event listeners, children, etc.
+    return createElement('button', context.data, context.children)
+  }
+})
+```
+
+By passing `context.data` as the second argument to `createElement`, we are passing down any attributes or event listeners used on `my-functional-button`. It's so transparent, in fact, that events don't even require the `.native` modifier.
+
 ### `slots()` vs `children`
 
 You may wonder why we need both `slots()` and `children`. Wouldn't `slots().default` be the same as `children`? In some cases, yes - but what if you have a functional component with the following children?