documentation: add git:// transport security notice

documentation: add git:// transport security notice

The fact that the git:// transport does no authentication is easily
overlooked.  For example, DNS poisoning may result in fetching from
somewhere that was not intended.

Add a brief security notice to the "GIT URLS" section
of the documentation stating that the git transport should be used
with caution on unsecured networks.

Signed-off-by: Fraser Tweedale <frase@frase.id.au>
Signed-off-by: Junio C Hamano <gitster@pobox.com>