gulp docs in Simplified Chinese

Created by: lisposter

Hi, everyone, since I saw the "Spanish documentation", I began to translate the docs into Simplified Chinese, and it almost done now.

Like the Spanish docs, I have created a repo https://github.com/lisposter/gulp-docs-zh-cn for the docs in zh-cn.

From now, I plan to review all the docs, and keep sync with the English doc.