The bulk of this patch consists of a heavily butchered version of
tbdiff's README written by Thomas Rast and Thomas Gummerer, lifted from
https://github.com/trast/tbdiff.
Signed-off-by: NJohannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: NJunio C Hamano <gitster@pobox.com>