Document how we do embargoed releases

Whenever we fix critical vulnerabilities, we follow some sort of
protocol (e.g. setting a coordinated release date, keeping the fix under
embargo until that time, coordinating with packagers and/or hosting
sites, etc).

Similar in spirit to `Documentation/howto/maintain-git.txt`, let's
formalize the details in a document.

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