message: add a contributor
diff: the crate root. Public APIs which deal in digits, like `BigUint::from_slice`,
  will now always be base-`u32`.
  -**Contributors**: @clarcharr, @cuviper, @dodomorandi, @tspiteri
  +**Contributors**: @clarcharr, @cuviper, @dodomorandi, @tiehuis, @tspiteri
  [8]: https://github.com/rust-num/num-bigint/pull/8
  [22]: https://github.com/rust-num/num-bigint/pull/22
  