Add doc on bot ranges.
Add doc on bot ranges.

Change-Id: Id4e5cc39b669cca80d84e512a4d99f91a9cf61ff
Reviewed-on: https://skia-review.googlesource.com/71860
Reviewed-by: Eric Boren <0e499112533c8544f0505ea0d08394fb5ad7d8fa@google.com>
Commit-Queue: Ben Wagner <3ef7217be91069877d94f7907ce5479000772cd3@google.com>

google/skia-buildbot,google/skia-buildbot,google/skia-buildbot,google/skia-buildbot,google/skia-buildbot,google/skia-buildbot,google/skia-buildbot,google/skia-buildbot