add PasswordInput component

docs/examples-docs/password-input.md

@@ -0,0 +1,88 @@
+<script>
+export default {
+  data() {
+    return {
+      value: '',
+      showKeyboard: true
+    }
+  },
+
+  methods: {
+    onInput(key) {
+      this.value = (this.value + key).slice(0, 6);
+    },
+    onDelete() {
+      this.value = this.value.slice(0, this.value.length - 1);
+    }
+  }
+}
+</script>
+
+## PasswordInput 密码输入框
+密码输入框组件通常与 [数字键盘](/zanui/vue/component/number-keyboard) 组件配合使用
+
+### 使用指南
+``` javascript
+import { PasswordInput, NumberKeyBoard } from 'vant';
+
+Vue.component(PasswordInput.name, PasswordInput);
+Vue.component(NumberKeyBoard.name, NumberKeyBoard);
+```
+
+### 代码演示
+
+#### 基础用法
+
+:::demo 基础用法
+```html
+<!-- 密码输入框 -->
+<van-password-input
+  :value="value"
+  info="密码为 6 位数字"
+  @focus="showKeyboard = true"
+></van-password-input>
+
+<!-- 数字键盘 -->
+<van-number-keyboard
+  :show="showKeyboard"
+  @input="onInput"
+  @delete="onDelete"
+  @blur="showKeyboard = false"
+></van-number-keyboard>
+```
+
+```javascript
+export default {
+  data() {
+    return {
+      value: '',
+      showKeyboard: true
+    }
+  },
+
+  methods: {
+    onInput(key) {
+      this.value = (this.value + key).slice(0, 6);
+    },
+    onDelete() {
+      this.value = this.value.slice(0, this.value.length - 1);
+    }
+  }
+}
+```
+:::
+
+### API
+
+| 参数 | 说明 | 类型 | 默认值 | 可选值 |
+|-----------|-----------|-----------|-------------|-------------|
+| value | 密码值 | `String`  | `''` | - |
+| length | 密码长度 | `Number` | `6` | - |
+| info | 输入框下方提示 | `String` | - | - |
+| errorInfo | 输入框下方错误提示 | `String` | - | - |
+
+### Event
+
+| 事件名       | 说明      | 参数       |
+|-----------|-----------|-----------|
+| focus | 输入框聚焦时触发 | - |