Auto merge of #5395 - dwijnand:contributing-setup-cross-tests, r=alexcrichton
Auto merge of #5395 - dwijnand:contributing-setup-cross-tests, r=alexcrichton

add contributing notes for cross compile test setup

adding notes from @alexcrichton's help in https://gitter.im/rust-lang/cargo?at=5ada2eae2b9dfdbc3ab01f29.

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