[Document] add form components english document (#199)

* [Document] add english document of Checkbox

* [Document] add english document of Field

* [Document] add english document of NumberKeyboard

* [bugfix] NumberKeyboard should not dispaly title when title is empty

* [Document] add english document of PasswordInput

* [Document] add english document of Radio

* [document] add english document of Switch

* [bugfix] remove redundent styles in english document

* [Document] fix details

* fix Switch test cases

docs/examples-docs/en-US/switch.md

@@ -0,0 +1,112 @@
+<script>
+import Dialog from 'packages/dialog';
+
+export default {
+  data() {
+    return {
+      checked: true,
+      checked2: true
+    };
+  },
+
+  methods: {
+    onInput(checked) {
+      Dialog.confirm({
+        title: 'Confirm',
+        confirmButtonText: 'ok',
+        cancelButtonText: 'cancel',
+        message: 'Are you sure to toggle switch?'
+      }).then(() => {
+        this.checked2 = checked;
+      });
+    }
+  }
+};
+</script>
+
+## Switch
+
+### Install
+``` javascript
+import { Switch } from 'vant';
+
+Vue.component(Switch.name, Switch);
+```
+
+### Usage
+
+#### Basic Usage
+
+:::demo Basic Usage
+```html
+<van-switch v-model="checked" />
+```
+
+```javascript
+export default {
+  data() {
+    return {
+      checked: true
+    };
+  }
+};  
+```
+:::
+
+#### Disabled
+
+:::demo Disabled
+```html
+<van-switch v-model="checked" disabled />
+```
+:::
+
+#### Loading
+
+
+:::demo Loading
+```html
+<van-switch v-model="checked" loading />
+```
+:::
+
+#### Advanced usage
+:::demo Advanced usage
+```html
+<van-switch :value="checked2" @input="onInput" />
+```
+```js
+export default {
+  data() {
+    return {
+      checked2: true
+    };
+  },
+
+  methods: {
+    onInput(checked) {
+      Dialog.confirm({
+        title: 'Confirm',
+        message: 'Are you sure to toggle switch?'
+      }).then(() => {
+        this.checked2 = checked;
+      });
+    }
+  }
+};  
+```
+:::
+
+### API
+
+| Attribute | Description | Type | Default | Accepted Values |
+|-----------|-----------|-----------|-------------|-------------|
+| v-model | Check status of Switch | `Boolean` | `false` | - |
+| loading | Whether to show loading icon | `Boolean`  | `false` | - |
+| disabled | Disable switch | `Boolean`  | `false` | - |
+
+### Event
+
+| Event | Description | Parameters |
+|-----------|-----------|-----------|
+| change | Triggered when check status changed | checked: is switch checked |