Add DM package doc.go.
Add DM package doc.go.

This just adds documentation for the package organization to help reviewers
map out which code does what.

R=dnj@chromium.org, estaab@chromium.org, vadimsh@chromium.org
BUG=533019

Review URL: https://codereview.chromium.org/1442013002

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