Add Chinese Simplified translation.

https://crowdin.net/project/inkpad/zh-CN

Translators:
Pillow Tse <https://github.com/xiezhhw>
Zhang Yungui <https://github.com/rhcad>
Hu Haonan <https://github.com/huhaonan>
Translate the help page into Chinese by xiezhhw and rhcad.
Merge pull request #100 from rhcad/chinese

Add Chinese Simplified translation by xiezhhw and rhcad.