This adds a hint for end users that run into a common failure mode where
ripgrep won't search any files because they have a `*` rule in their
`$HOME/.gitignore`.
Fixes#815
This commit cleans up the README and splits portions of it out into
a user guide (GUIDE.md) and a FAQ (FAQ.md). The README now provides a
small list of documentation "quick" links to various parts of the docs.
This commit also does a few other minor touchups.
