Update LUCI_CONTEXT.md to describe "luciexe" section.
Update LUCI_CONTEXT.md to describe "luciexe" section.

R=9f116ddb1b24f6fc1916a676eb17161b6c07dfc1@chromium.org

Bug: 939368
Change-Id: I42587b6c5a81ff54f4faad675382dc10656dde0b
Reviewed-on: https://chromium-review.googlesource.com/c/infra/luci/luci-py/+/1745860
Auto-Submit: Robbie Iannucci <40f3d43a28ebae3cb819288542e1c84d73d962d5@chromium.org>
Commit-Queue: Vadim Shtayura <9f116ddb1b24f6fc1916a676eb17161b6c07dfc1@chromium.org>
Reviewed-by: Vadim Shtayura <9f116ddb1b24f6fc1916a676eb17161b6c07dfc1@chromium.org>

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