Added documentation section about how to start logstash as marathon app
Added documentation section about how to start logstash as marathon app

suppandi/logstash,kensipe/logstash,alex-glv/logstash,frankscholten/logstash,mesosphere/logstash,suppandi/logstash,frankscholten/logstash,kensipe/logstash,kensipe/logstash,drewrobb/logstash,drewrobb/logstash,suppandi/logstash,frankscholten/logstash,kensipe/logstash,drewrobb/logstash,mesosphere/logstash,alex-glv/logstash,drewrobb/logstash,mesos/logstash,frankscholten/logstash,alex-glv/logstash,mesos/logstash,suppandi/logstash,mesosphere/logstash,alex-glv/logstash,mesosphere/logstash