[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/checkbox.md

@@ -0,0 +1,139 @@
+<script>
+export default {
+  data() {
+    return {
+      checkbox1: true,
+      checkbox2: true,
+      list: [
+        'a',
+        'b',
+        'c'
+      ],
+      result: ['a', 'b']
+    };
+  }
+};
+</script>
+
+## Checkbox
+
+### Install
+``` javascript
+import { Checkbox, CheckboxGroup } from 'vant';
+
+Vue.component(Checkbox.name, Checkbox);
+Vue.component(CheckboxGroup.name, CheckboxGroup);
+```
+
+### Usage
+
+#### Basic Usage
+
+:::demo Basic Usage
+```html
+<van-checkbox v-model="checkbox1">Checkbox 1</van-checkbox>
+```
+
+```javascript
+export default {
+  data() {
+    return {
+      checkbox1: true
+    };
+  }
+};
+```
+:::
+
+#### Disabled
+
+:::demo Disabled
+```html
+<van-checkbox v-model="checkbox2" disabled>Checkbox 2</van-checkbox>
+```
+:::
+
+#### CheckboxGroup
+When Checkboxes are inside a CheckboxGroup, the checked checkboxes's name is an array and bound with CheckboxGroup by v-model.
+
+:::demo CheckboxGroup
+```html
+<van-checkbox-group v-model="result">
+  <van-checkbox
+    v-for="(item, index) in list"
+    :key="index"
+    :name="item"
+  >
+    Checkbox {{ item }}
+  </van-checkbox>
+</van-checkbox-group>
+```
+
+```javascript
+export default {
+  data() {
+    return {
+      list: ['a', 'b', 'c'],
+      result: ['a', 'b']
+    };
+  },
+
+  watch: {
+    result(val) {
+      console.log(val);
+    }
+  }
+};
+```
+:::
+
+#### Inside a Cell
+
+:::demo Inside a Cell
+```html
+<van-checkbox-group v-model="result">
+  <van-cell-group>
+    <van-cell v-for="(item, index) in list" :key="index">
+      <van-checkbox :name="item">Checkbox{{ item }}</van-checkbox>
+    </van-cell>
+  </van-cell-group>
+</van-checkbox-group>
+```
+
+```javascript
+export default {
+  data() {
+    return {
+      list: ['a', 'b', 'c'],
+      result: ['a', 'b']
+    };
+  }
+};
+```
+:::
+
+### Checkbox API
+
+| Attribute | Description | Type | Default | Accepted Values |
+|-----------|-----------|-----------|-------------|-------------|
+| name | Checkbox name | `Boolean`  | `false` | - |
+| disabled | Diable checkbox | `Boolean`  | `false` | - |
+| shape | Checkbox shape | `String`  | `round` | `square` |
+
+### CheckboxGroup API
+
+| Attribute | Description | Type | Default | Accepted Values |
+|-----------|-----------|-----------|-------------|-------------|
+| disabled | Disable all checkboxes | `Boolean` | `false` | - |
+
+### Checkbox Event
+
+| Event | Description | Parameters |
+|-----------|-----------|-----------|
+| change | Triggered when value changed | current value |
+
+### CheckboxGroup Event
+
+| Event | Description | Parameters |
+|-----------|-----------|-----------|
+| change | Triggered when value changed | current value |