Update REDMINE.md
Update REDMINE.md

Adding a line about custom domain for Google Apps. It might clear some problem people might have in the future.
hasegaw/docker-redmine,joshuacox/docker-redmine,joshuacox/docker-redmine,ekulos/docker-redmine,patope/docker-redmine,EvgenyKarataev/docker-redmine,sameersbn/docker-redmine,vladm/docker-redmine,sameersbn/docker-redmine,sameersbn/docker-redmine,otsuka512/docker-redmine,otsuka512/docker-redmine,tigerich/docker-redmine,EvgenyKarataev/docker-redmine,otsuka512/docker-redmine,ekulos/docker-redmine,tigerich/docker-redmine,patope/docker-redmine,hasegaw/docker-redmine,vladm/docker-redmine