Added a note regarding custom ssh_port

If you do use a custom SSH port on your gitlab box, you do not need to change the `ssh_port` value in config/gitlab.yml if you have an entry in your ~/.ssh/config file. For example:

    Host git.domain.com
        HostName git.domain.com
        Port     12345

You will still be able to clone using nice pretty URLs: `git@git.domain.com:group/project.git`