message: [Doc] Card: update demo
diff: @@ -13,28 +13,44 @@ Vue.use(Card);
  ```html
  <van-card
  + num="2"
  + price="2.00"
  title="Title"
  desc="Description"
  + thumb="https://img.yzcdn.cn/vant/t-thirt.jpg"
  +/>
  +```
  +
  +### Discount Info
  +
  +```html
  +<van-card
  num="2"
  + tag="Tag"
  price="2.00"
  - :thumb="imageURL"
  + title="Title"
  + desc="Description"
  + origin-price="10.00"
  + thumb="https://img.yzcdn.cn/vant/t-thirt.jpg"
  />
  ```
  -### Advanced Usage
  +### Custom Content
  Use slot to custom content.
  ```html
  <van-card
  num="2"
  - tag="Tag"
  title="Title"
  desc="Description"
  price="2.00"
  - :thumb="imageURL"
  - origin-price="10.00"
  + thumb="https://img.yzcdn.cn/vant/t-thirt.jpg"
  >
  + <div slot="tags">
  + <van-tag plain type="danger">Tag</van-tag>
  + <van-tag plain type="danger">Tag</van-tag>
  + </div>
  <div slot="footer">
  <van-button size="mini">Button</van-button>
  <van-button size="mini">Button</van-button>
  