Auto merge of #5343 - dwijnand:contributing-fmt, r=matklad
authorbors <bors@rust-lang.org>
Wed, 11 Apr 2018 11:44:20 +0000 (11:44 +0000)
committerbors <bors@rust-lang.org>
Wed, 11 Apr 2018 11:44:20 +0000 (11:44 +0000)
add a rustfmt detail to CONTRIBUTING.md

stealing from the info in https://github.com/rust-lang/cargo/pull/5301#issuecomment-380389555


Trivial merge