feat: 🎨 add placeholder for the custom input

Author: shengyuan

Diff for: index.html

@@ -372,7 +372,7 @@
     </div>
     <div id="main02" class="panel hide">
       <div class="input-group">
-        <input type="text" class="input-group-input" id="customMessage">
+        <input type="text" class="input-group-input" id="customMessage" placeholder="輸入文字">
         <button class="input-group-button">傳送</button>
       </div>
       <div id="btn0n" msg="btn0" class="btn"></div>