Update Security FAQ

Update the security FAQ with information about Chrome signout and bug
severity guidelines.

[email protected]

Change-Id: Iace8601bebd5dc2efd2753dc9862a9aafb1f0360
Reviewed-on: https://chromium-review.googlesource.com/814756
Reviewed-by: Eric Lawrence <[email protected]>
Reviewed-by: Chris Palmer <[email protected]>
Commit-Queue: Eric Lawrence <[email protected]>
Cr-Commit-Position: refs/heads/master@{#522588}
1 file changed