How/where to de document non-public information?

[ChristianKuehnel]

When creating a proposal for the LLVM board in #32 I noticed that we will need a place to store sensitive information (e.g. pricing).

Requirements:

• not publicly visible
• permissions can be configured per document or folder
• we can at least store textual documents and PDFs
• optional: storing arbitrary files would be nice to have
• ideally something, the LLVM community is familiar with
• It supports some review/collaboration process (e.g. see history and edits, create comments, ...)