Add changelog for 5.6.0

Change-Id: Ib3177028cf1cbd124ebf1449d5e00039f38b1a92
Reviewed-by: Tuukka Turunen <tuukka.turunen@theqtcompany.com>
Reviewed-by: Oswald Buddenhagen <oswald.buddenhagen@theqtcompany.com>
Reviewed-by: Lars Knoll <lars.knoll@theqtcompany.com>
Reviewed-by: Frederik Gladhorn <frederik.gladhorn@theqtcompany.com>