improve spark on yarn doc(issus 3629 on jira)

Right now  running-on-yarn.md starts off with a big list of config options, and only then tells you how to submit an app. It would be better to put that part and the packaging part first, and the config options only at the end. Besides I add some explainations about yarn-cluster and yarn-client mode 

https://issues.apache.org/jira/browse/SPARK-3629