Add PPSP.

[rektide]

"How Hypercore Works"[1] describes the underlying data structure of Hypercore, which is the flat-tree discussed in RFC 7574 PPSP. Add this link.

[1] https://dat-data.readthedocs.io/en/latest/hyperdrive_spec/#how-hypercore-works

[joehand]

I think having this link in the docs is sufficient. The RFC is not related to dat directly, so I don't see the need to have this in this repo.