Documentation: describe incremental MIDX format

Documentation: describe incremental MIDX format

Prepare to implement incremental multi-pack indexes (MIDXs) over the
next several commits by first describing the relevant prerequisites
(like a new chunk in the MIDX format, the directory structure for
incremental MIDXs, etc.)

The format is described in detail in the patch contents below, but the
high-level description is as follows.

Incremental MIDXs live in $GIT_DIR/objects/pack/multi-pack-index.d, and
each `*.midx` within that directory has a single "parent" MIDX, which is
the MIDX layer immediately before it in the MIDX chain. The chain order
resides in a file 'multi-pack-index-chain' in the same directory.

Signed-off-by: Taylor Blau <me@ttaylorr.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>