Add a CONTRIBUTING.md

As the title says, create a contributing.md file for the sake of this repository. The rules written in this file should tell a user how coding conventions and general guidelines can be used to contribute to this repository.

在仓库根目录创建 CONTRIBUTING.md。在编写编码约定和一般贡献指南之前，先审查仓库现有的约定；当文件存在并说明用户如何为 FirstRustGame 做出贡献时，即视为完成。

由索引模型根据 Issue 内容生成。