added support for after_commit callback to README
added support for after_commit callback to README

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