added note

index.html

@@ -52,6 +52,7 @@ <h1>光棱坦克显形</h1>
         <input type="text" id="imageUrlInput" placeholder="输入图片URL">
         <button id="loadImageButton">加载图片</button>
     </div>
+    <div><small>(暂时并不支持所有URL, 例如百度贴吧)</small></div>
     <img id="originalImage" src="" alt="原始图像" crossorigin="anonymous" style="display:none;">
     <canvas id="canvas"></canvas>