04-open.md: Mention Bitbucket's free private repositories

From the current https://bitbucket.org/plans:

  All plans include:
  * Unlimited private repos
  ...

I think the main reason to use common hosting sites like GitHub and
Bitbucket is increased visibility or access to features (like the web
interface and bug trackers).

Personally, I wouldn't trust a third party host to protect my private
data when I can easily host it myself.  I'm not sure how widely shared
this paranoia is ;), but I'd certainly want to talk to lawyers before
hosting patient medical records in a private repository on Bitbucket.