Update installation instructions to use EXTERNAL_URL variable for simpler installation process

Related to https://gitlab.com/gitlab-org/omnibus-gitlab/issues/2619 .

Currently, users need to edit gitlab.rb file after installation to set a URL for their GitLab instance and then run gitlab-ctl reconfigure to apply it. With https://gitlab.com/gitlab-org/omnibus-gitlab/merge_requests/1839 , we added an option to pass EXTERNAL_URL with the installation command so that users no longer need to do that. That means, installation is now as simple as

1. Run the script to setup repository
2. EXTERNAL_URL="http://GitLabIsSoAwesome.com" apt-get install gitlab-ce
3. Enjoy

We need to update the installation instructions to reflect this change.

/cc @joshlambert @marin