Create FAQ-like documentation

[calebcartwright]

I've been thinking it'd be good to pull together a document that contains info on / pointers to commonly discussed/referenced topics that seem to pop up regularly in GH issues, Discord, etc.

Some suggested items an FAQ-like doc could speak to (or have links/pointers to for things documented elsewhere):

• Strategies for incrementally adopting rustfmt in existing projects
• Disabling rustfmt/preventing rustfmt from running in a project
• config option stabilization process
• relationship between rustfmt & cargo fmt
• perhaps the versioning strategy for rustfmt?

Thoughts?