docs(Field): add basic usage document

src/form/README.md

@@ -13,6 +13,48 @@ Vue.use(Form);
 
 ### Basic Usage
 
+```html
+<van-form @submit="onSubmit" @failed="onFailed">
+  <van-field
+    v-model="username"
+    name="Username"
+    label="Username"
+    placeholder="Username"
+    :rules="[{ required: true, message: 'Username is required' }]"
+  />
+  <van-field
+    v-model="password"
+    type="password"
+    name="Password"
+    label="Password"
+    placeholder="Password"
+    :rules="[{ required: true, message: 'Password is required' }]"
+  />
+  <div style="margin: 16px;">
+    <van-button round block type="info">Submit</van-button>
+  </div>
+</van-form>
+```
+
+```js
+export default {
+  data() {
+    return {
+      username: '',
+      password: '',
+    };
+  },
+  methods: {
+    onSubmit(values) {
+      console.log('submit', values);
+    },
+    onFailed(errorInfo) {
+      console.log('failed', errorInfo);
+    },
+  },
+}
+```
+
 ## API
 
 ### Props