Update README.md for Rust 1.59

[futile]

Since Cargo.toml now specifies strip = ..., and strip is only available in cargo >=1.59, specify that at least this version is required.

Description

Update README.md with the info that at least Rust 1.59 is required.

Ran into this in nixpkgs, where nushell 0.60 doesn't build with Rust 1.58 anymore, see: NixOS/nixpkgs#165473

Tests

Make sure you've run and fixed any issues with these commands:

• cargo fmt --all -- --check to check standard code formatting (cargo fmt --all applies these changes)
• cargo clippy --all --all-features -- -D warnings -D clippy::unwrap_used -A clippy::needless_collect to check that you're using the standard code style
• cargo build; cargo test --all --all-features to check that all the tests pass

Disclaimer: I haven't run any of the checks mentioned above (just edited the file on github).

[sophiajt]

Thanks!