Add builder description.

Bug: 1066815
Change-Id: I2d2bba2f5b446d352a16744f6270e01a65c2d88a
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/2249204
Auto-Submit: Weiwei Lin <weiweilin@google.com>
Reviewed-by: Vadim Shtayura <vadimsh@chromium.org>
Commit-Queue: Vadim Shtayura <vadimsh@chromium.org>