Add some notes about updating an asset in infra/bots/

Change-Id: Id667ad7409477f364c722387b3407ba283804413
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/635883
Reviewed-by: Kevin Lubick <kjlubick@google.com>
Commit-Queue: James Godfrey-Kittle <jamesgk@google.com>
2 files changed