docs/os: add hardening guide for CoreOS
docs/os: add hardening guide for CoreOS

Many people have asked for best practices for hardening CoreOS. This
documents some of the basic practices to check out when considering
CoreOS.

joshix/docs,endocode/coreos-docs,coreos/docs,mjg59/docs,robszumski/docs,atxwebs/docs,jonboulle/docs