Rephrase documentation to emphasize building.
Rephrase documentation to emphasize building.

TBR=nodir@chromium.org
TEST=None

Bug:
Change-Id: I0ae9881b520f7932465d8f9e28f2c3c7a5cb30fe
Reviewed-on: https://chromium-review.googlesource.com/636475
Reviewed-by: Jao-ke Chin-Lee <5de1a387cc854ff3d04fab3d66d4e77a58511771@chromium.org>
Commit-Queue: Jao-ke Chin-Lee <5de1a387cc854ff3d04fab3d66d4e77a58511771@chromium.org>

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