doc: technical details about the index file format
doc: technical details about the index file format

This bases on the original work by Robin Rosenberg.

Signed-off-by: Robin Rosenberg <2564e1a542077ae1498767dd7104ac9a06908e7a@dewire.com>
Signed-off-by: Nguyễn Thái Ngọc Duy <024f400cd820cb2ddbe54c7fecfd01e45f30bb96@gmail.com>
Signed-off-by: Junio C Hamano <a6723cc3f76163bf7adb636a73ac3b0ceb3e6b9b@pobox.com>

destenson/git,destenson/git,destenson/git,destenson/git,destenson/git,destenson/git,destenson/git,destenson/git