Add comment to markdown on how to run EarlGrey tests.

EarlGrey tests are run differently than other iOS targets, so this
should be document.

Bug: 
Change-Id: I112108008a85f810b7e96fb0241b1c8277ec9ea9
Reviewed-on: https://chromium-review.googlesource.com/567918
Reviewed-by: Colin Blundell <[email protected]>
Commit-Queue: Mike Baxley <[email protected]>
Cr-Commit-Position: refs/heads/master@{#485951}
1 file changed