Add Chinese Simplified translation by xiezhhw and rhcad.

We have translated the help page and localizable strings into Chinese, and tested on iPad too.

The main apple store page translated into Chinese can be [download](https://raw.githubusercontent.com/rhcad/Inkpad/appstore_chs/Help/AppStore.txt).

Translators:
Pillow Tse <https://github.com/xiezhhw>
Zhang Yungui <https://github.com/rhcad>