Document how to get a specific revision when rebaselining.

This is helpful when using the trybots.

R=epoger@google.com

Review URL: https://codereview.chromium.org/25653008

git-svn-id: http://skia.googlecode.com/svn/trunk@11643 2bbb7eff-a529-9590-31e7-b0007b416f81
1 file changed