Fixes #3941 - Explain how configure yum repositories to add Rudder and its GPG key
Merge pull request #20 from nperron/bug_3941/doc/explain_how_configure_yum_to_accept_gpgkeys

Fixes #3941 - Explain how configure yum repositories to add Rudder and its GPG key
Merge branch 'branches/rudder/2.4' into branches/rudder/2.6

Conflicts:
	10_install_server/13_install_root_server_centos_rhel.txt
	11_install_agent/20_install_agent_centos_rhel.txt
	12_upgrade/20_upgrade_centos_rhel.txt