message: Update CONTRIBUTING.md
  Since code is rejected during CI if `rustfmt` complains. Update the docs regarding that fact.
diff: @@ -81,6 +81,10 @@ of helpers to minimize boilerplate.
  installed locally, ignore the cross-compile test failures or disable them by
  using `CFG_DISABLE_CROSS_TESTS=1 cargo test`. Note that some tests are enabled
  only on `nightly` toolchain. If you can, test both toolchains.
  +* All code is expected to comply with the formatting suggested by `rustfmt`;
  +discrepancy is considered an error by CI, so a pull request with unsound
  +formatting will not be accepted. You can use `rustup component add rustfmt-preview`
  +to install `rustfmt` and use `cargo fmt` to automatically format your code.
  * Push your commits to GitHub and create a pull request against Cargo's
  `master` branch.
  