We've actually stored legal documents on Github in the past (think contracts, proposals, employee handbook, invoices...) as Markdown files. It felt very strange though to collaborate around files through Git. The `PULL > COMMIT > PUSH` workflow works well for a lot of things, not necessarily well when you're preparing a contract.
We've actually built a tool around Dropbox for that very use case. Happy to share it here when it's ready end of this month.