feat(image): 增加跨平台示例

docs/api/image.md

@@ -12,3 +12,21 @@
 
 <!-- CUSTOMTYPEJSON.Image.example -->
 
+**提示**
+
+微信小程序不支持 `new Image()` 写法来绘制，需要通过跨平台写法 `CanvasContext.drawImage()`, 示例如下:
+
+```js
+uni.createCanvasContextAsync({
+  id: 'canvas',
+  component: this, // setup模式使用 getCurrentInstance().proxy
+  success: (context : CanvasContext) => {
+    const renderingContext = context.getContext('2d')!;
+    const image = context.createImage();
+    image.src = "/static/logo.png";
+    image.onload = () => {
+      renderingContext.drawImage(image, 0, 0, 100, 100);
+    }
+  }
+})
+```