[Rust] Document our approach to unsafe code in README #18413

It would be helpful to document the project's stance on the use of unsafe code in a prominent location such as in the top-level README so that we can refer people to this when questions are asked about this.

Reporter: Andy Grove / @andygrove
Assignee: Jorge Leitão / @jorgecarleitao

PRs and other links:

• GitHub Pull Request #8901

_{Note: This issue was originally created as ARROW-10889. Please see the migration documentation for further details.}