hacking: Improve 'git send-email' documentation

hacking: Improve 'git send-email' documentation

For the benefit of first time contributors, we point out that 'git
send-email' might have to be installed separately; however, we omit
the fact that some configuration will likely be needed before it
can successfully deliver patches to the mailing list.

Some minor tweaks to the existing contents are included as well.

Signed-off-by: Andrea Bolognani <abologna@redhat.com>
Reviewed-by: John Ferlan <jferlan@redhat.com>