message: [Doc] Popup: add get-container demo
diff: @@ -12,9 +12,11 @@ Vue.use(Popup);
  ### Basic Usage
  -Popup is located in the middle of the screen by default
  -
  ```html
  +<van-botton type="primary" @click="showPopup">
  + Show Popup
  +</van-button>
  +
  <van-popup v-model="show">Content</van-popup>
  ```
  @@ -24,6 +26,12 @@ export default {
  return {
  show: false
  }
  + },
  +
  + methods: {
  + showPopup() {
  + this.show = true;
  + }
  }
  };
  ```
  @@ -33,9 +41,45 @@ export default {
  Use `position` prop to set popup display position
  ```html
  -<van-popup v-model="show" position="top" :overlay="false">
  - Content
  -</van-popup>
  +<van-popup
  + v-model="show"
  + position="top"
  + :style="{ height: '20%' }"
  +/>
  +```
  +
  +### Get Container
  +
  +Use `get-container` prop to specify mount location
  +
  +```html
  +<!-- mount to body -->
  +<van-popup
  + v-model="show"
  + get-container="body"
  +/>
  +
  +<!-- mount to #app -->
  +<van-popup
  + v-model="show"
  + get-container="#app"
  +/>
  +
  +<!-- Specify the mount location by function -->
  +<van-popup
  + v-model="show"
  + :get-container="getContainer"
  +/>
  +```
  +
  +```js
  +export default {
  + methods: {
  + getContainer() {
  + return document.querySelector('.my-container');
  + }
  + }
  +}
  ```
  ## API
  