Write a codefmt FAQ

Codefmt raises a few questions like [why not just use formatprg](https://www.reddit.com/r/vim/comments/2uab1g/codefmt_a_plugin_for_formatting_code_using/co6lljx) that we should officially answer in the docs, pbb in README.md.

I added a bit of FAQ content to https://github.com/google/vim-codefmt/wiki/Formatter-Integration-Guide. Part or all of that can be merged into a main FAQ.

查看 README.md 和 Formatter Integration Guide 中现有的 FAQ 内容，包括关于为什么使用 codefmt 而不是 formatprg 的链接问题。将相关答案整合到 README 中的主 FAQ，确保 FAQ 涵盖用户可能会提出的问题。

由索引模型根据 Issue 内容生成。