Mielink portable mode marker.

这个文件的存在会让 Mielink 把配置、订阅、设置和日志写到同目录的
userdata\ 里，而不是 %APPDATA%。删除本文件即回退为使用 %APPDATA%
（已写入 userdata\ 的数据不会自动搬回去）。

While this file exists, Mielink stores its config, subscriptions,
settings and logs in the userdata\ folder next to the executable
instead of %APPDATA%. Delete this file to fall back to %APPDATA%
(data already written to userdata\ is not migrated back).
