Auto merge of #5395 - dwijnand:contributing-setup-cross-tests, r=alexcrichton
authorbors <bors@rust-lang.org>
Fri, 20 Apr 2018 18:43:05 +0000 (18:43 +0000)
committerbors <bors@rust-lang.org>
Fri, 20 Apr 2018 18:43:05 +0000 (18:43 +0000)
add contributing notes for cross compile test setup

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


Trivial merge