docs: add note about disabling direct port mappings
docs: add note about disabling direct port mappings

[#157675252](https://www.pivotaltracker.com/story/show/157675252)

cloudfoundry-incubator/diego-release,cloudfoundry-incubator/diego-release,cloudfoundry-incubator/diego-release,cloudfoundry-incubator/diego-release