Auto merge of rust-lang#5331 - dwijnand:contributing-fmt, r=matklad

Tweak rustfmt details in CONTRIBUTING.md

Closes rust-lang#5329

CONTRIBUTING.md

@@ -81,10 +81,9 @@ 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.
+* All code changes are expected to comply with the formatting suggested by `rustfmt`.
+You can use `rustup component add rustfmt-preview` to install `rustfmt` and use
+`cargo fmt` on the changed files to automatically format your code.
 * Push your commits to GitHub and create a pull request against Cargo's
 `master` branch.