Provide rsync access to GNU ELPA for mirroring it

[Chunyang Xu]

Hello emacs-devel!

The emacs-china user group is running an ELPA mirror [1] to provide
faster and more reliable connection for Chinese Emacs users.
Currently, it simply downloads all files through HTTP, it works well
for more than one month, however, mirroring through rsync is obviously
easier and more efficient. So please consider providing rsync access
to Gnu elpa.

[1] https://github.com/emacs-china/elpa

Thanks.

Chunyang