docs(Form): add Field Type docs

src/form/README.md

@@ -55,6 +55,146 @@ export default {
 }
 ```
 
+### Field Type - Switch
+
+```html
+<van-field name="switch" label="Switch">
+  <van-switch v-model="switchChecked" slot="input" size="20" />
+</van-field>
+```
+
+```js
+export default {
+  data() {
+    return {
+      switchChecked: false,
+    };
+  },
+};
+```
+
+### Field Type - Checkbox
+
+```html
+<van-field name="checkbox" label="Checkbox">
+  <van-checkbox v-model="checkbox" slot="input" shape="square" />
+</van-field>
+<van-field name="checkboxGroup" label="CheckboxGroup">
+  <van-checkbox-group
+    v-model="checkboxGroup"
+    slot="input"
+    direction="horizontal"
+  >
+    <van-checkbox name="1" shape="square">Checkbox 1</van-checkbox>
+    <van-checkbox name="2" shape="square">Checkbox 2</van-checkbox>
+  </van-checkbox-group>
+</van-field>
+```
+
+```js
+export default {
+  data() {
+    return {
+      checkbox: false,
+      checkboxGroup: [],
+    };
+  },
+};
+```
+
+### Field Type - Radio
+
+```html
+<van-field name="radio" label="Radio">
+  <van-radio-group v-model="radio" direction="horizontal" slot="input">
+    <van-radio name="1">Radio 1</van-radio>
+    <van-radio name="2">Radio 2</van-radio>
+  </van-radio-group>
+</van-field>
+```
+
+```js
+export default {
+  data() {
+    return {
+      radio: '1',
+    };
+  },
+};
+```
+
+### Field Type - Stepper
+
+```html
+<van-field name="stepper" label="Stepper">
+  <van-stepper v-model="stepper" slot="input" />
+</van-field>
+```
+
+```js
+export default {
+  data() {
+    return {
+      stepper: 1,
+    };
+  },
+};
+```
+
+### Field Type - Rate
+
+```html
+<van-field name="rate" label="Rate">
+  <van-rate v-model="rate" slot="input" />
+</van-field>
+```
+
+```js
+export default {
+  data() {
+    return {
+      rate: 3,
+    };
+  },
+};
+```
+
+### Field Type - Slider
+
+```html
+<van-field name="slider" label="Slider">
+  <van-slider v-model="slider" slot="input" />
+</van-field>
+```
+
+```js
+export default {
+  data() {
+    return {
+      slider: 50,
+    };
+  },
+};
+```
+
+### Field Type - Uploader
+
+```html
+<van-field name="uploader" label="Uploader">
+  <van-uploader v-model="uploader" slot="input" multiple max-count="2" />
+</van-field>
+```
+
+```js
+export default {
+  data() {
+    return {
+      uploader: [{ url: 'https://img.yzcdn.cn/vant/leaf.jpg' }],
+    };
+  },
+};
+```
+
 ## API
 
 ### Props