Add a small note to win_cross.md

Bug: 
Change-Id: I390df5f47a9a735441d7f5180ce0ba132ebc08f3
Reviewed-on: https://chromium-review.googlesource.com/743684
Reviewed-by: Nico Weber <thakis@chromium.org>
Commit-Queue: Nico Weber <thakis@chromium.org>
Cr-Commit-Position: refs/heads/master@{#512511}
1 file changed