Add simple measure against non-targeted spam.

index.html

@@ -229,6 +229,11 @@
       <label for="description">Description:</label><br>
       <textarea id="description" name="description" cols="80" rows="20" required></textarea><br>
 
+      <div style="display:none;">
+        <label for="url" style="width: 18ch;">Leave this empty:</label>
+        <input type="text" id="url" name="url">
+      </div>
+
       <input type="submit">
     </form>