[cv][doc] update top-level readme.
[cv][doc] update top-level readme.

Perfcrastination at its best :P

R=robertocn, yiwzhang

No-Try: true
Change-Id: I03fbddc3a9e72aa2ff6d1d0d33be2b0ec32e67e6
Reviewed-on: https://chromium-review.googlesource.com/c/infra/luci/luci-go/+/3119507
Commit-Queue: Andrii Shyshkalov <a30c74fa30536fe7ea81ed6dec202e35e149e1fd@google.com>
Reviewed-by: Yiwei Zhang <50b2a565e2e78c292794832469a30ce5abc9959c@google.com>

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