docs(Field): add basic usage document

src/form/demo/BasicUsage.vue

@@ -0,0 +1,61 @@
+<template>
+  <demo-block :title="$t('basicUsage')">
+    <van-form @submit="onSubmit" @failed="onFailed">
+      <van-field
+        v-model="username"
+        name="username"
+        :label="$t('username')"
+        :rules="[{ required: true, message: $t('requireUsername') }]"
+        :placeholder="$t('username')"
+      />
+      <van-field
+        v-model="password"
+        type="password"
+        name="password"
+        :label="$t('password')"
+        :rules="[{ required: true, message: $t('requirePassword') }]"
+        :placeholder="$t('password')"
+      />
+      <div style="margin: 16px;">
+        <van-button type="info" round block>{{ $t('submit') }}</van-button>
+      </div>
+    </van-form>
+  </demo-block>
+</template>
+
+<script>
+export default {
+  i18n: {
+    'zh-CN': {
+      submit: '提交',
+      username: '用户名',
+      password: '密码',
+      requireUsername: '请填写用户名',
+      requirePassword: '请填写密码',
+    },
+    'en-US': {
+      submit: 'Submit',
+      username: 'Username',
+      password: 'Password',
+      requireUsername: 'Username is required',
+      requirePassword: 'Password is required',
+    },
+  },
+
+  data() {
+    return {
+      username: '',
+      password: '',
+    };
+  },
+
+  methods: {
+    onSubmit(values) {
+      console.log('submit', values);
+    },
+    onFailed(errorInfo) {
+      console.log('failed', errorInfo);
+    },
+  },
+};
+</script>