add contributing notes for cross compile test setup
add contributing notes for cross compile test setup

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