update README for Release 3.0
update README for Release 3.0

mrkeng/alerta,skob/alerta,0312birdzhang/alerta,guardian/alerta,mrkeng/alerta,skob/alerta,skob/alerta,skob/alerta,guardian/alerta,mrkeng/alerta,0312birdzhang/alerta,0312birdzhang/alerta,guardian/alerta,guardian/alerta,mrkeng/alerta