[Doc] add some english documents (#220)

* [bugfix] Checkbox border render error in weixin browser

* [bugfix] TreeSelect dependency path error

* [bugfix] Swipe should clear autoplay timer when destroyed

docs/examples-docs/en-US/nav-bar.md

@@ -0,0 +1,51 @@
+## NavBar
+
+### Install
+``` javascript
+import { NavBar } from 'vant';
+
+Vue.component(NavBar.name, NavBar);
+```
+
+### Usage
+
+#### Basic Usage
+
+:::demo Basic Usage
+```html
+<van-nav-bar
+  title="Title"
+  left-text="Back"
+  right-text="Button"
+  left-arrow
+/>
+```
+:::
+
+#### Advanced Usage
+
+:::demo Advanced Usage
+```html
+<van-nav-bar title="Title" left-text="Back" left-arrow>
+  <van-icon name="search" slot="right" />
+</van-nav-bar>
+```
+:::
+
+
+### API
+| Attribute | Description | Type | Default | Accepted Values |
+|-----------|-----------|-----------|-------------|-------------|
+| title | Title | `String` | `''` | - |
+| left-text | Left Text | `String` | `''` | - |
+| right-text | Right Text | `String` | `''` | - |
+| left-arrow | Whether to show left arrow | `Boolean` | `false` | - |
+| fixed | Whether to fixed top | `Boolean` | `false` | - |
+
+### Slot
+
+| name | Description |
+|-----------|-----------|
+| title | Custom title |
+| left | Custom left side content |
+| right | Custom right side content |