[Release tools] Add instructions how to close the auto-roller.
[Release tools] Add instructions how to close the auto-roller.

R=machenbach@chromium.org
BUG=494641
LOG=N
NOTRY=true

Review URL: https://codereview.chromium.org/1217173003

Cr-Commit-Position: 972c6d2dc6dd5efdad1377c0d224e03eb8f276f7@{#29404}

UniversalFuture/moosh,UniversalFuture/moosh,UniversalFuture/moosh,UniversalFuture/moosh