documentation: add tutorial for first contribution

documentation: add tutorial for first contribution

This tutorial covers how to add a new command to Git and, in the
process, everything from cloning git/git to getting reviewed on the
mailing list. It's meant for new contributors to go through
interactively, learning the techniques generally used by the git/git
development community.

Reviewed-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Reviewed-by: Jonathan Tan <jonathantanmy@google.com>
Signed-off-by: Emily Shaffer <emilyshaffer@google.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>