filter-branch: add an example how to add ACKs to a range of commits

filter-branch: add an example how to add ACKs to a range of commits

When you have to add certain lines like ACKs (or for that matter,
Signed-off-by:s) to a range of commits starting with HEAD, you might
be tempted to use 'git rebase -i -10', but that is a waste of your
time.

It is better to use 'git filter-branch' with an appropriate message
filter, and this commit adds an example how to do so to
filter-branch's man page.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>