update contributing a bit re contributing code (#2903)
update contributing a bit re contributing code (#2903)

* CONTRIBUTING: document starter_task label

* CONTRIBUTING: PR should work travis python versions

* CONTRIBUTING: move PR guidelines at the top

* CONTRIBUTING: no need to open a new pr after review

airbnb/caravel,BRD-CD/superset,FrederichCheng/incubator-superset,timifasubaa/incubator-superset,dmigo/incubator-superset,nekia/incubator-superset-dev,dmigo/incubator-superset,nekia/incubator-superset-dev,asdf2014/superset,apache/incubator-superset,asdf2014/superset,airbnb/superset,apache/incubator-superset,alanmcruickshank/superset-dev,zhouyao1994/incubator-superset,asdf2014/superset,mistercrunch/panoramix,alanmcruickshank/superset-dev,BRD-CD/superset,airbnb/caravel,airbnb/superset,zhouyao1994/incubator-superset,mistercrunch/panoramix,massgov/incubator-superset,airbnb/superset,timifasubaa/incubator-superset,zhouyao1994/incubator-superset,SingTel-DataCo/incubator-superset,alanmcruickshank/superset-dev,FrederichCheng/incubator-superset,nekia/incubator-superset-dev,mistercrunch/panoramix,asdf2014/superset,airbnb/superset,apache/incubator-superset,massgov/incubator-superset,airbnb/caravel,BRD-CD/superset,timifasubaa/incubator-superset,FrederichCheng/incubator-superset,timifasubaa/incubator-superset,SingTel-DataCo/incubator-superset,SingTel-DataCo/incubator-superset,SingTel-DataCo/incubator-superset,dmigo/incubator-superset,apache/incubator-superset,zhouyao1994/incubator-superset,nekia/incubator-superset-dev,mistercrunch/panoramix,BRD-CD/superset,apache/incubator-superset,FrederichCheng/incubator-superset,dmigo/incubator-superset,airbnb/caravel,massgov/incubator-superset,zhouyao1994/incubator-superset,alanmcruickshank/superset-dev,massgov/incubator-superset