doc: add technical design doc for large object promisors

Let's add a design doc about how we could improve handling liarge blobs
using "Large Object Promisors" (LOPs). It's a set of features with the
goal of using special dedicated promisor remotes to store large blobs,
and having them accessed directly by main remotes and clients.

Signed-off-by: Christian Couder <[email protected]>
Signed-off-by: Junio C Hamano <[email protected]>