Document the package.
Change-Id: I770c14d6339b75ea6827280c23171c3584dc1071
Reviewed-on: https://go-review.googlesource.com/c/tools/+/674977
Reviewed-by: Robert Findley <rfindley@google.com>
Reviewed-by: Alan Donovan <adonovan@google.com>
Auto-Submit: Jonathan Amsterdam <jba@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
