[luci-config] Add pointer to ui/README.md in README.md
[luci-config] Add pointer to ui/README.md in README.md

Bug: 850750
Change-Id: Ia80c1a5c27c9ab7339e56187a1be76f3e215f19a
Reviewed-on: https://chromium-review.googlesource.com/1092031
Reviewed-by: Sergey Berezin <002a79d3522c3eaf360691f06af29425bafc4480@chromium.org>
Commit-Queue: smut <95deb7aa15d4c615299e47e7688551456568b1fb@google.com>

luci/luci-py,luci/luci-py,luci/luci-py,luci/luci-py