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.