Merge #20

20: Note a few alternative crates in README.md r=cuviper a=cuviper

This presents a table of some common alternatives to this crate which may have better performance.  I'm not including any specific performance comparisons, since that will surely vary on different workloads, and will change as each crate evolves.  I want to keep this impartial, listing only simple objective facts about each.  Corrections welcome!

cc crate owners: @Aatch, @doomrobo, @fizyk20, @tspiteri

README.md

@@ -29,3 +29,22 @@ 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 |
+| [`ramp`]         | Apache-2.0     | nightly   | rust and inline assembly |
+| [`rug`]          | LGPL-3.0+      | 1.18      | bundles [GMP] via [`gmp-mpfr-sys`] |
+| [`rust-gmp`]     | MIT            | stable?   | links to [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