message: note a few alternative crates
diff: @@ -29,3 +29,23 @@ Release notes are available in [RELEASES.md](RELEASES.md).
  ## Compatibility
  The `num-bigint` crate is tested for rustc 1.8 and greater.
  +
  +## Alternatives
  +
  +While `num-bigint` strives for good performance in pure Rust code, other
  +crates may offer better performance with different trade-offs. The following
  +table offers a brief comparison to a few alternatives.
  +
  +| Crate | License | Min rustc | Implementation |
  +| :--------------- | :------------- | :-------- | :------------- |
  +| **`num-bigint`** | MIT/Apache-2.0 | 1.8 | pure rust |
  +| [`gmp-mpfr-sys`] | LGPL-3.0+ | 1.13 | unsafe bindings for [GMP] |
  +| [`ramp`] | Apache-2.0 | nightly | rust and inline assembly |
  +| [`rug`] | LGPL-3.0+ | 1.18 | safe interface using `gmp-mpfr-sys` |
  +| [`rust-gmp`] | MIT | stable? | safe bindings for [GMP] |
  +
  +[GMP]: https://gmplib.org/
  +[`gmp-mpfr-sys`]: https://crates.io/crates/gmp-mpfr-sys
  +[`rug`]: https://crates.io/crates/rug
  +[`rust-gmp`]: https://crates.io/crates/rust-gmp
  +[`ramp`]: https://crates.io/crates/ramp
  