DOC Update README to add call to action to join mailing list.
DOC Update README to add call to action to join mailing list.

YuepengGuo/zipline,umuzungu/zipline,humdings/zipline,kmather73/zipline,otmaneJai/Zipline,CDSFinance/zipline,jimgoo/zipline-fork,morrisonwudi/zipline,CDSFinance/zipline,joequant/zipline,jordancheah/zipline,florentchandelier/zipline,humdings/zipline,bartosh/zipline,grundgruen/zipline,wubr2000/zipline,Scapogo/zipline,Scapogo/zipline,MonoCloud/zipline,zhoulingjun/zipline,quantopian/zipline,magne-max/zipline-ja,keir-rex/zipline,YuepengGuo/zipline,morrisonwudi/zipline,DVegaCapital/zipline,alphaBenj/zipline,ronalcc/zipline,alphaBenj/zipline,AlirezaShahabi/zipline,chrjxj/zipline,otmaneJai/Zipline,magne-max/zipline-ja,gwulfs/zipline,umuzungu/zipline,dmitriz/zipline,iamkingmaker/zipline,quantopian/zipline,iamkingmaker/zipline,grundgruen/zipline,StratsOn/zipline,enigmampc/catalyst,cmorgan/zipline,enigmampc/catalyst,sketchytechky/zipline,euri10/zipline,ronalcc/zipline,zhoulingjun/zipline,semio/zipline,michaeljohnbennett/zipline,semio/zipline,stkubr/zipline,ChinaQuants/zipline,keir-rex/zipline,michaeljohnbennett/zipline,aajtodd/zipline,florentchandelier/zipline,AlirezaShahabi/zipline,bartosh/zipline,sketchytechky/zipline,wubr2000/zipline,aajtodd/zipline,kmather73/zipline,nborggren/zipline,wilsonkichoi/zipline,jimgoo/zipline-fork,dkushner/zipline,dkushner/zipline,gwulfs/zipline,DVegaCapital/zipline,joequant/zipline,nborggren/zipline,ChinaQuants/zipline,StratsOn/zipline,wilsonkichoi/zipline,dmitriz/zipline,cmorgan/zipline,MonoCloud/zipline,euri10/zipline,chrjxj/zipline,jordancheah/zipline,stkubr/zipline