message: Add a release note for Shr
diff: # Release 0.2.0 (pending)
  - :warning: [num-bigint now requires rustc 1.15 or greater][23].
  +- :warning: [`Shr for BigInt` now rounds down][8] rather than toward zero,
  + matching the behavior of the primitive integers for negative values.
  **Contributors**: @cuviper
  +[8]: https://github.com/rust-num/num-bigint/pull/8
  [23]: https://github.com/rust-num/num-bigint/pull/23
  