[MiKTeX] Add new repository into Miktex offical repository list

[z. <zhu...@qq...>]

Hi, 
   I'm from China and the current repositories in the official list are too slow to access. Could you add `https://mirrors.tuna.tsinghua.edu.cn/CTAN/` into the official list? I've tried `mpm --set-repository=https://mirrors.tuna.tsinghua.edu.cn/CTAN/systems/win32/miktex/tm/packages/` to change the repository. However, it failed.


   Thanks.


Mingzhou