Mention existing redirection in migration docs

TBR=fdoray@chromium.org
NOTRY=True

Bug: 
Change-Id: I4050913dd0173642652312e0911529b6be26a9cd
Reviewed-on: https://chromium-review.googlesource.com/529747
Reviewed-by: Gabriel Charette <gab@chromium.org>
Commit-Queue: Gabriel Charette <gab@chromium.org>
Cr-Commit-Position: refs/heads/master@{#478358}
1 file changed