feat(picture.md): @.avg_color (#893)

docs/misc/picture.md

@@ -9,7 +9,7 @@
 | width    | 图片最大限制宽度 |                  |
 | high     | 图片最大限制高度 |                  |
 | quality  | 图片质量百分比   | 仅限webp         |
-| format   | 图片格式         | 仅限png/jpg/webp |
+| format   | 图片格式         | 仅限png/jpg/webp/[avg_color](#avg_color格式说明) |
 
 **示例：**
 
@@ -41,4 +41,40 @@ https://i1.hdslb.com/bfs/archive/e5fff1472bad1c0c6bcb3004205f9be23b58ffc0.jpg@.w
 
 https://i1.hdslb.com/bfs/archive/e5fff1472bad1c0c6bcb3004205f9be23b58ffc0.jpg@1q.webp
 
-![](https://i1.hdslb.com/bfs/archive/e5fff1472bad1c0c6bcb3004205f9be23b58ffc0.jpg@1q.webp)
+![](https://i1.hdslb.com/bfs/archive/e5fff1472bad1c0c6bcb3004205f9be23b58ffc0.jpg@1q.webp)
+
+
+## avg_color格式说明
+
+当图片格式化输出格式为`avg_color`时
+
+> \*.hdslb.com/bfs/\*/\*.*@\*.avg_color
+
+*请求方式：GET*
+
+**json回复：**
+
+根对象：
+
+| 字段   | 类型 | 内容         | 备注   |
+| ------ | ---- | ----------- | ------ |
+| RGB    | str  | 平均颜色值   | HEX    |
+
+**示例：**
+
+获取 https://i1.hdslb.com/bfs/archive/e5fff1472bad1c0c6bcb3004205f9be23b58ffc0.jpg 的平均颜色值
+
+```shell
+curl 'https://i1.hdslb.com/bfs/archive/e5fff1472bad1c0c6bcb3004205f9be23b58ffc0.jpg@.avg_color'
+```
+
+<details>
+<summary>查看响应示例：</summary>
+
+```json
+{
+    "RGB": "#7d6f6c"
+}
+```
+
+</details>