README improvements and better geard example

@bparees It does, when rendered, see https://github.com/VojtechVitek/ruby-19-centos/blob/0bef78adbcf008bc2352986c3e5c61f97e73f9ad/README.md. I think it's just not obvious from this diff. Do you think it's OK, or should I rebase and mention this explicitly?