Document how forks can use different set of jobs and add
a big warning so no new configuration is added to this
file.

Suggested-by: Daniel Berrangé <berrange@redhat.com>
Signed-off-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Reviewed-by: Alex Bennée <alex.bennee@linaro.org>
Message-Id: <20210525082556.4011380-4-f4bug@amsat.org>