[Doc] add some english documents (#220)

* [bugfix] Checkbox border render error in weixin browser

* [bugfix] TreeSelect dependency path error

* [bugfix] Swipe should clear autoplay timer when destroyed

docs/examples-docs/en-US/waterfall.md

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+## Waterfall
+
+### Install
+
+#### 全局注册
+
+`Waterfall`引入后就自动全局安装。如果需要，可以再次手动安装：
+
+```js
+import Vue from 'vue';
+import { Waterfall } from 'vant';
+
+Vue.use(Waterfall);
+```
+
+#### 局部注册
+
+如果你只是想在某个组件中使用`Waterfall`，可以在对应组件中注册`Waterfall`指令，这样只能在你注册的组件中使用`Waterfall`：
+
+```js
+import { Waterfall } from 'vant';
+
+export default {
+  directives: {
+    WaterfallLower: Waterfall('lower'),
+    WaterfallUpper: Waterfall('upper')
+  }
+};
+```
+
+### Usage
+
+<script>
+import { Waterfall } from 'packages';
+
+export default {
+  data() {
+    return {
+      list: [1, 2, 3, 4, 5, 6, 7, 8, 9, 10],
+      loading: false,
+      finished: false
+    };
+  },
+  directives: {
+    WaterfallLower: Waterfall('lower'),
+    WaterfallUpper: Waterfall('upper')
+  },
+  methods: {
+    loadMore() {
+      if (this.list.length >= 50) {
+        this.finished = true;
+        return;
+      }
+
+      this.loading = true;
+      setTimeout(() => {
+        let lastNumber = this.list[this.list.length - 1];
+        for (let i = 0; i < 5; i ++) {
+          lastNumber += 1;
+          this.list.push(lastNumber);
+        }
+        this.loading = false;
+      }, 200);
+    }
+  },
+  computed: {
+    isWaterfallDisabled() {
+      return this.loading || this.finished;
+    }
+  }
+};
+</script>
+
+<style>
+.demo-waterfall {
+  ul {
+    max-height: 360px;
+    overflow: scroll;
+    border-top: 1px solid #e5e5e5;
+  }
+  li {
+    line-height: 50px;
+    border-bottom: 1px solid #e5e5e5;
+    background: #fff;
+    text-align: center;
+  }
+  .page-desc {
+    padding: 5px 0;
+    line-height: 1.4;
+    font-size: 14px;
+    text-align: center;
+    color: #666;
+  }
+}
+</style>
+
+#### Basic Usage
+使用 `v-waterfall-lower` 监听滚动到达底部，并执行相应函数。若是函数执行中需要异步加载数据，可以将 `waterfall-disabled` 指定的值置为 false，禁止 `v-waterfall-lower` 监听滚动事件
+
+注意：`waterfall-disabled` 传入的是 vue 对象中表示是否禁止瀑布流触发 key 值，类型是字符串
+:::demo Basic Usage
+```html
+<p class="page-desc">当即将滚动到元素底部时，会自动加载更多</p>
+<ul
+  v-waterfall-lower="loadMore"
+  waterfall-disabled="isWaterfallDisabled"
+  waterfall-offset="400">
+  <li v-for="(item, index) in list">{{ item }}</li>
+</ul>
+```
+:::
+
+### API
+
+| Attribute | Description | Type | Default | Accepted Values |
+|-----------|-----------|-----------|-------------|-------------|
+| v-waterfall-lower | 滚动到底部, 触发执行的函数 | `Function` | - | |
+| v-waterfall-upper | 滚动到顶部, 触发执行的函数 | `Function` | - | |
+| waterfall-disabled | 在 vue 对象中表示是否禁止瀑布流触发的 key 值 | `String` | - | |
+| waterfall-offset | 触发瀑布流加载的阈值 | `Number` | `300` | |
+