message: Update release notes from 0.2.6
diff: +# Release 0.2.6 (2020-01-27)
  +
  +- [Fix the promotion of negative `isize` in `BigInt` assign-ops][133].
  +
  +**Contributors**: @cuviper, @HactarCE
  +
  +[133]: https://github.com/rust-num/num-bigint/pull/133
  +
  # Release 0.2.5 (2020-01-09)
  - [Updated the `autocfg` build dependency to 1.0][126].
  