Improve README.md

The README.md file has been converted to Markdown and been slightly extended.

Here are some further changes I'd like to see:

Not sure if it's still portable to Hugs. We claim it is, but I doubt it.

The section about "Using binary". I think this section is not too helpful. It could be more structured of the alternatives and with links to the relevant haddocks.

Deriving binary instance section. This is outdated and missleading. We can now generate instances with GHCs generics. A small example of how to do so, or link to the haddocks, would be better suiting.

首先阅读 README.md，重点关注 Hugs 的可移植性声明、“Using binary”部分以及 deriving 部分。编辑前检查当前的 Haddocks 和 GHC generics 指南；当 README 准确描述可移植性、通过相关链接介绍使用替代方案，并展示或链接到现代的实例生成方式时，即视为完成。

由索引模型根据 Issue 内容生成。