Add Omnibus package download details to downloads page

I realize you'd like us all to use the package bash script and yum but some of us can't because servers are blocked from external internet access. Please add details to the download page on how to do manual download. Would be something like:

curl https://packages.gitlab.com/gitlab/gitlab-ce/packages/el/6/gitlab-ce-7.11.4~omnibus-1.x86_64.rpm
sudo rpm -i gitlab-ce-7.11.4~omnibus-1.x86_64.rpm

Obviously EL 7 & Ubuntu would have slightly different directions.

Or even simplier:

1. Download correct package from https://packages.gitlab.com/gitlab/gitlab-ce
2. Install using dpkg -i gitlab-ce-XXX.deb (for Debian/Ubuntu) or rpm -i gitlab-ce-XXX.rpm