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Doc: how to add a new chromium LUCI builder.

Change-Id: Ie5901f9f1188fb271fcf8c2d6482c2fbd490e742
Reviewed-on: https://chromium-review.googlesource.com/1130457
Commit-Queue: John Budorick <jbudorick@chromium.org>
Reviewed-by: Eric Foo <efoo@chromium.org>
Cr-Commit-Position: refs/heads/master@{#576520}