Document permissions needed to edit a dashboard

[toolness]

While I was investigating #130, I realized that there's something a bit confusing about this package, which is that editing them takes a user to the Django admin edit view for a Dashboard. This seems to imply that a user needs to at least have is_staff=True, since users who aren't staff can't access the Django admin at all (at least, I think they can't). It seems like this should be documented.

Beyond that, though, I'm confused by what kinds of permissions a user whom we want to allow editing of dashboards should have. Because they're being directed to a Django admin edit view, it feels like the user should be required to have "change" permission for Dashboard models, but they don't actually appear to need to... maybe this should be documented?