Improve README
Improve README

Fix typo
Link to the actual Contributing file
massgov/incubator-superset,mistercrunch/panoramix,zhouyao1994/incubator-superset,SingTel-DataCo/incubator-superset,dmigo/incubator-superset,airbnb/superset,giaosudau/caravel,jeromecn/caravel_viz_full,airbnb/caravel,robert-digit/superset,alanmcruickshank/superset-dev,zhouyao1994/incubator-superset,timifasubaa/incubator-superset,alanmcruickshank/superset-dev,ArijanaBrlek/superset,timifasubaa/incubator-superset,dmigo/incubator-superset,georgeke/caravel,wbsljh/caravel,airbnb/caravel,BRD-CD/superset,giaosudau/caravel,georgeke/caravel,jeromecn/caravel_viz_full,jeromecn/caravel_viz_full,wbsljh/caravel,zhouyao1994/incubator-superset,apache/incubator-superset,woel0007/caravel,tienhv/caravel,francisliyy/caravel-aidp,airbnb/superset,airbnb/caravel,robert-digit/superset,SingTel-DataCo/incubator-superset,alanmcruickshank/superset-dev,FrederichCheng/incubator-superset,francisliyy/caravel-aidp,asdf2014/superset,francisliyy/caravel-aidp,woel0007/caravel,apache/incubator-superset,wbsljh/caravel-aidp,thebucknerlife/caravel,wbsljh/caravel,francisliyy/caravel-aidp,thebucknerlife/caravel,woel0007/caravel,robert-digit/superset,SingTel-DataCo/incubator-superset,wbsljh/caravel-aidp,timifasubaa/incubator-superset,alanmcruickshank/superset-dev,BRD-CD/superset,airbnb/caravel,BRD-CD/superset,dmigo/incubator-superset,mistercrunch/panoramix,apache/incubator-superset,apache/incubator-superset,FrederichCheng/incubator-superset,wbsljh/caravel,airbnb/superset,zhouyao1994/incubator-superset,mistercrunch/panoramix,tienhv/caravel,FrederichCheng/incubator-superset,wbsljh/caravel-aidp,dmigo/incubator-superset,airbnb/superset,tienhv/caravel,massgov/incubator-superset,ArijanaBrlek/superset,giaosudau/caravel,robert-digit/superset,georgeke/caravel,massgov/incubator-superset,asdf2014/superset,zhouyao1994/incubator-superset,wbsljh/caravel-aidp,woel0007/caravel,ArijanaBrlek/superset,ArijanaBrlek/superset,mistercrunch/panoramix,massgov/incubator-superset,asdf2014/superset,FrederichCheng/incubator-superset,nekia/incubator-superset-dev,thebucknerlife/caravel,nekia/incubator-superset-dev,nekia/incubator-superset-dev,timifasubaa/incubator-superset,asdf2014/superset,apache/incubator-superset,tienhv/caravel,SingTel-DataCo/incubator-superset,BRD-CD/superset,thebucknerlife/caravel,nekia/incubator-superset-dev,georgeke/caravel,giaosudau/caravel,jeromecn/caravel_viz_full