SubmittingPatches: clarify 'git-contacts' location

SubmittingPatches: clarify 'git-contacts' location

Use a dash ("git-contacts", not "git contacts") because the script is
not installed as part of "git" toolset. This also puts the script on
one line, which should make it easier to grep for with a loose search
query, such as

    $ git grep git.contacts Documentation

Also add a footnote to describe where the script is located, to help
readers who may not be familiar with such "contrib" scripts (and how
they are not accessible with the usual "git <subcommand>" syntax).

Signed-off-by: Linus Arver <linusa@google.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>