[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/radio.md

@@ -0,0 +1,131 @@
+<script>
+export default {
+  data() {
+    return {
+      radio1: '1',
+      radio2: '2',
+      radio3: '1',
+      radio4: '1'
+    };
+  }
+};
+</script>
+
+## Radio
+
+### Install
+``` javascript
+import { Radio } from 'vant';
+
+Vue.component(Radio.name, Radio);
+```
+
+### Usage
+
+#### Basic Usage
+Use `v-model` to bind check status of radio. The value will be set to the name of radio when radio get checked.
+
+:::demo Basic Usage
+```html
+<div class="van-radios">
+  <van-radio name="1" v-model="radio1">Radio 1</van-radio>
+  <van-radio name="2" v-model="radio1">Radio 2</van-radio>
+</div>
+```
+```javascript
+export default {
+  data() {
+    return {
+      radio1: '1'
+    }
+  }
+};
+```
+:::
+
+#### Disabled
+
+:::demo Disabled
+```html
+<div class="van-radios">
+  <van-radio name="1" v-model="radio2" disabled>Disabled</van-radio>
+  <van-radio name="2" v-model="radio2" disabled>Disabled and checked</van-radio>
+</div>
+```
+
+```javascript
+export default {
+  data() {
+    return {
+      radio2: '2'
+    }
+  }
+};
+```
+:::
+
+#### RadioGroup
+When Radios are inside a RadioGroup, the checked radio's name is bound with CheckboxGroup by `v-model`.
+
+:::demo RadioGroup
+```html
+<div class="van-radios">
+  <van-radio-group v-model="radio3">
+    <van-radio name="1">Radio 1</van-radio>
+    <van-radio name="2">Radio 2</van-radio>
+  </van-radio-group>
+</div>
+```
+  
+```javascript
+export default {
+  data() {
+    return {
+      radio3: '1'
+    }
+  }
+};
+```
+:::
+
+#### With Cell
+
+:::demo With Cell
+```html
+<van-radio-group v-model="radio4">
+  <van-cell-group>
+    <van-cell><van-radio name="1">Radio 1</van-radio></van-cell>
+    <van-cell><van-radio name="2">Radio 2</van-radio></van-cell>
+  </van-cell-group>
+</van-radio-group>
+```
+
+```javascript
+export default {
+  data() {
+    return {
+      radio4: '1'
+    }
+  }
+};
+```
+:::
+
+### Radio API
+
+| Attribute | Description | Type | Default | Accepted Values |
+|-----------|-----------|-----------|-------------|-------------|
+| disabled | Diable radio | `Boolean` | `false` | - |
+| name | Radio name | `Boolean` | `false` | - |
+
+### RadioGroup API
+
+| Attribute | Description | Type | Default | Accepted Values |
+|-----------|-----------|-----------|-------------|-------------|
+| disabled | Diable all radios | `Boolean` | `false` | - |
+
+### RadioGroup Event
+
+| Event | Description | Parameters |
+|-----------|-----------|-----------|
+| change | Triggered when value changed | current value |