Better add some placeholders texts to firebase example.

on page https://github.com/vuejs/vuejs.org/blob/master/src/examples/firebase.md

From

``` html
    <input v-model="newUser.name">
    <input v-model="newUser.email">
```

to

``` html
    <input v-model="newUser.name" placeholder="Username">
    <input v-model="newUser.email" placeholder="demo@email.com">
```

Like this: https://jsfiddle.net/qiansen1386/2d3htmpr/89/
