Added missing su option documentation to `README.md`.
Added missing su option documentation to `README.md`.


Former-commit-id: 2a89fde2b2314d53509be0a3635cfc433b257251
yo61/puppet-logrotate,covermymeds/puppet-logrotate,apa-it/puppet-logrotate,apa-it/puppet-logrotate,ncsutmf/puppet-logrotate,yo61/puppet-logrotate,karolisc/puppet-logrotate,cwood/puppet-logrotate,covermymeds/puppet-logrotate,ncsutmf/puppet-logrotate,karolisc/puppet-logrotate,icann-dns/puppet-logrotate,zeromind/puppet-logrotate,icann-dns/puppet-logrotate,cwood/puppet-logrotate,zeromind/puppet-logrotate,zeromind/puppet-logrotate,apa-it/puppet-logrotate,cwood/puppet-logrotate,covermymeds/puppet-logrotate,karolisc/puppet-logrotate,ncsutmf/puppet-logrotate,icann-dns/puppet-logrotate,yo61/puppet-logrotate