[new feature] add Circle component (#608)

docs/markdown/en-US/circle.md

@@ -0,0 +1,69 @@
+## Circle
+
+### Install
+``` javascript
+import { Circle } from 'vant';
+
+Vue.use(Circle);
+```
+
+### Usage
+
+#### Basic Usage
+
+```html
+<van-circle
+  v-model="currentRate"
+  :rate="30"
+  :speed="100"
+  :text="text"
+/>
+```
+
+``` javascript
+export default {
+  data() {
+    return {
+      currentRate: 0
+    };
+  },
+  computed: {
+    text() {
+      return this.currentRate.toFixed(0) + '%'
+    }
+  }
+};
+```
+
+#### Custom style
+
+```html
+<van-circle
+  v-model="currentRate"
+  color="#13ce66"
+  fill="#fff"
+  size="120px"
+  layer-color="#eee"
+  :text="text"
+  :rate="rate"
+  :speed="100"
+  :clockwise="false"
+  :stroke-width="60"
+/>
+```
+
+
+### API
+
+| Attribute | Description | Type | Default | Accepted Values |
+|-----------|-----------|-----------|-------------|-------------|
+| v-model | Current rate | `Number` | - | - |
+| rate | Target rate | `Number` | `100` | - |
+| size | Circle size | `String` | `100px` | - |
+| color | Progress bar color | `String` | `#38f` | - |
+| layer-color | Layer color | `String` | `#fff` | - |
+| fill | Fill color | `String` | `none` | - |
+| speed | Animate speed（rate/s）| `Number` | - | - |
+| text | Text | `String` | - | - |
+| stroke-width | Stroke width | `Number` | `40` | - |
+| clockwise | Is clockwise | `Boolean` | `true` | - |