[cv] improve README.md.
[cv] improve README.md.

R=robertocn, yiwzhang

Change-Id: I43197b5e7f3cdac7e1c6f163c31df982f672a647
Reviewed-on: https://chromium-review.googlesource.com/c/infra/luci/luci-go/+/3157913
Commit-Queue: Andrii Shyshkalov <a30c74fa30536fe7ea81ed6dec202e35e149e1fd@google.com>
Auto-Submit: Andrii Shyshkalov <a30c74fa30536fe7ea81ed6dec202e35e149e1fd@google.com>
Reviewed-by: Roberto Carrillo <b31161e510fbb37f8f70fdcde11530d2bae88470@chromium.org>

luci/luci-go,luci/luci-go,luci/luci-go,luci/luci-go,luci/luci-go,luci/luci-go