Fix gsubtreed missing __init__.py .
Fix gsubtreed missing __init__.py .

This was broken when documentation was moved to markdown https://codereview.chromium.org/1245643003

BUG=520403
R=iannucci@chromium.org,akuegel@chromium.org

Review URL: https://codereview.chromium.org/1288323002

nicko96/Chrome-Infra,nicko96/Chrome-Infra,nicko96/Chrome-Infra,nicko96/Chrome-Infra,nicko96/Chrome-Infra