simplify and improve README
simplify and improve README

Licenser/relx,bsmr-erlang/relx,RJ/relx,lrascao/relx,ingwinlu/relx,tsloughter/relx,priestjim/relx,zugolosian/relx,umatomba/relx,ethercrow/relx,syl20bnr/relx,erlware/relx,timclassic/relx,bitwalker/relx