added README for migration from version 3 to 4
added README for migration from version 3 to 4

ASnow/aasm,yui-knk/aasm,Funfun/aasm,intfrr/aasm,hspazio/aasm,aasm/aasm,oswaldoferreira/aasm,allspiritseve/aasm,nickpellant/aasm,awsmsrc/aasm,reidmorrison/aasm,pskrz/aasm,xieyunzi/aasm,Aryk/aasm,artofhuman/aasm,joshsoftware/aasm,bilby91/aasm,HoyaBoya/aasm,luxola/aasm