README.md

CLI commands which are mentioned in the readme are now formatted with
the Markdown code syntax to make the documentation more readable.

Signed-off-by: Benjamin Dopplinger <b.dopplinger@gmail.com>
Reviewed-by: Matthieu Moy <Matthieu.Moy@imag.fr>
Signed-off-by: Junio C Hamano <gitster@pobox.com>