Point to design documents

[mih]

http://docs.datalad.org/en/latest/design/index.html is a place where information on the basic design of datalad might show up. There are three PRs proposed ATM. If, and only if, this becomes an established place for this kind of info. It would make sense to point to it for an alternative, possibly more technical level of description.

Alternatively, we could start extracting useful bits and place them in a dedicated chapter somewhere here. This cannot be a simple copy operation, because a substantial amount of incorrect/outdated information is present there.