Improve overview / introduction in README / docs.rs

[epage]

From @passcod in #74

For other docs-related things, see my comment on the assert_fs issue as this crate is structured very similarly and suffers from most of the same issues.

I'm assuming referring to

Foremost, while the rustdoc is good as a reference, the upfront documentation is severely lacking. The readme is practically useless both for figuring out what this is (beyond the title, which is only helpful if one knows what all the words mean), and figuring out what it can do (and hence, whether it can be used for one's usecase). Going to the docs.rs page in some hope of more is equally discouraging (and also non-obvious from github as it's only linked through the badge), as the crate top-level documentation is an extremely succinct "overview" that doesn't really explain anything.

See assert-rs/assert_fs#46