Documentation/gitpacking.txt: initial commit

Documentation/gitpacking.txt: initial commit

Introduce a new manual page, gitpacking(7) to collect useful information
about advanced packing concepts in Git.

In future commits in this series, this manual page will expand to
describe the new pseudo-merge bitmaps feature, as well as include
examples, relevant configuration bits, use-cases, and so on.

Outside of this series, this manual page may absorb similar pieces from
other parts of Git's documentation about packing.

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