Complete documents.

[elrond-g]

• Preface include intro, change log and so on.
• Agreement document includes code style and develop tools.
• Develop Guide

Welcome to propose document contents.

[YunzhiYike]

Give me

[elrond-g]

https://github.com/elrond-g/seata-php/tree/master_docs

@YunzhiYike 之前我写了一些文档，但是不全，并且分支的 commit 流程上好像出了点问题。

我重新拉个分支把内容组织一下，再提pr

[YunzhiYike]

@elrond-g 好