docs/zh_CN: add Alex Shi as Chinese documentation maintainer

It's kind of fun to learn and tranlate kernel docs into Chinese. I'd
like to help people on this work. And willing to be the 'blamed' people
if sth wrong in this area. :)

Signed-off-by: Alex Shi <alex.shi@linux.alibaba.com>
Cc: Harry Wei <harryxiyou@gmail.com>
Cc: Jonathan Corbet <corbet@lwn.net>
Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Cc: linux-doc@vger.kernel.org
Cc: linux-kernel@vger.kernel.org
Signed-off-by: Jonathan Corbet <corbet@lwn.net>