Add comment on why need to run `git clean` before deploy.
Add comment on why need to run `git clean` before deploy.

SkygearIO/skygear-server,rickmak/skygear-server,SkygearIO/skygear-server,rickmak/skygear-server,rickmak/skygear-server,SkygearIO/skygear-server,SkygearIO/skygear-server