CONTRIBUTING.adoc: document how to refer to a commit in commit messages

CONTRIBUTING.adoc: document how to refer to a commit in commit messages

This is what I already do, I would like it to make it an official
procedure.  What I want to avoid is having commits referred to just
using their hashes, which is meaningless for humans.

Change-Id: I47b5044d74d3d10e2b953fe7ad291b4468391da2
Signed-off-by: Simon Marchi <simon.marchi@efficios.com>
Reviewed-on: https://review.lttng.org/c/babeltrace/+/13379
Reviewed-by: Philippe Proulx <eeppeliteloop@gmail.com>