add a rustfmt detail to CONTRIBUTING.md
add a rustfmt detail to CONTRIBUTING.md

rustyhorde/cargo,tshepang/cargo,vojtechkral/cargo,tsurai/cargo,tshepang/cargo,rustyhorde/cargo,tsurai/cargo,tsurai/cargo,nipunn1313/cargo,rustyhorde/cargo,rust-lang/cargo,tshepang/cargo,rustyhorde/cargo,vojtechkral/cargo,nipunn1313/cargo,eminence/cargo,tsurai/cargo,rust-lang/cargo,vojtechkral/cargo,nipunn1313/cargo,vojtechkral/cargo,tsurai/cargo,rust-lang/cargo,eminence/cargo,tshepang/cargo,eminence/cargo,rust-lang/cargo,nipunn1313/cargo,eminence/cargo