Update README

Add an example of the usage of the `韻鏡位置2字頭()` function.

Author: ayaka14732

README.md

@@ -27,6 +27,7 @@ Here, we use the character 直 as an example, which is located at the 42th 轉,
 ```javascript
 const 當前韻鏡位置 = new 韻鏡位置(42, 15, 7); // 「直」
 console.log(當前韻鏡位置.描述); // 外轉第四十二圖·舌音第三位·入聲位·韻鏡三等
+console.log(韻鏡位置2字頭(當前韻鏡位置)); // 直
 ```
 
 ### Converting 音韻地位 to 韻鏡位置