Improve arrow's crate's readme on crates.io

[alamb]

Is your feature request related to a problem or challenge? Please describe what you are trying to do.
Right now, the documentation on crates.io is very technical and immediately starts into developer guide specific details

https://crates.io/crates/arrow

Describe the solution you'd like
The overview / summary of the crate should be clear, point at the main arrow docs / repo, and explain what Arrow is and can be used for. The developer details should be moved into Developer.md