Major readme overhaul for apache httpd

I have updated the README documentation for how to harden the production service of gitlab through httpd.  I may in the future cover firewalling in the README however I felt that was a little too far out of the scope for this README.  So I basically touched topics which directly relate to setting up a secure `httpd` instance for a production GitLab configuration.

This is to compliment your request from PR #125 where @axilleas mentioned this info would be useful in the README.