Documenting new installer option --foreman-ipa-authentication

This is a documentation for https://github.com/theforeman/puppet-foreman/pull/199
Good catch. It needs to be either
http://copr-fe.cloud.fedoraproject.org/coprs/adelton/identity_demo/
or
http://copr.fedoraproject.org/coprs/adelton/identity_demo/