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>

Documentation/Makefile

@@ -73,6 +73,7 @@ SP_ARTICLES += howto/maintain-git
 API_DOCS = $(patsubst %.txt,%,$(filter-out technical/api-index-skel.txt technical/api-index.txt, $(wildcard technical/api-*.txt)))
 SP_ARTICLES += $(API_DOCS)
 
+TECH_DOCS += MyFirstContribution
 TECH_DOCS += SubmittingPatches
 TECH_DOCS += technical/hash-function-transition
 TECH_DOCS += technical/http-protocol