update readme with supported providers
update readme with supported providers

tsmolka/packer,KohlsTechnology/packer,tas50/packer,smaato/packer,quantang/packer,smaato/packer,pecigonzalo/packer,tsmolka/packer,dayglojesus/packer,sean-/packer,tas50/packer,rnaveiras/packer,dave2/packer,pieter-lazzaro/packer,mitchellh/packer,taliesins/packer,bryson/packer,dave2/packer,bryson/packer,arizvisa/packer,mkuzmin/packer,paulmey/packer,tb3088/packer,jen20/packer,grubernaut/packer,sean-/packer,timsutton/packer,c22/packer,boumenot/packer,smaato/packer,ChrisLundquist/packer,mafrosis/packer,hashicorp/packer,pecigonzalo/packer,mafrosis/packer,arizvisa/packer,jen20/packer,bryson/packer,tas50/packer,arizvisa/packer,jen20/packer,rnaveiras/packer,atlassian/packer,dayglojesus/packer,mitchellh/packer,ricardclau/packer,p1-nico/packer,pvandervelde/packer,p1-nico/packer,p1-nico/packer,atlassian/packer,pieter-lazzaro/packer,pecigonzalo/packer,boumenot/packer,taliesins/packer,tsmolka/packer,quantang/packer,taliesins/packer,tb3088/packer,dave2/packer,timsutton/packer,sean-/packer,p1-nico/packer,pieter-lazzaro/packer,KohlsTechnology/packer,arizvisa/packer,dave2/packer,youhong316/packer,paulmey/packer,atlassian/packer,dayglojesus/packer,KohlsTechnology/packer,c22/packer,ricardclau/packer,markpeek/packer,atlassian/packer,ricardclau/packer,markpeek/packer,p1-nico/packer,mkuzmin/packer,mkuzmin/packer,timsutton/packer,timsutton/packer,youhong316/packer,ChrisLundquist/packer,quantang/packer,mitchellh/packer,taliesins/packer,KohlsTechnology/packer,pvandervelde/packer,pvandervelde/packer,ChrisLundquist/packer,rnaveiras/packer,arizvisa/packer,pvandervelde/packer,zhuzhih2017/packer,smaato/packer,dayglojesus/packer,jen20/packer,tb3088/packer,boumenot/packer,tas50/packer,youhong316/packer,mitchellh/packer,KohlsTechnology/packer,sean-/packer,routelastresort/packer,bryson/packer,dave2/packer,c22/packer,paulmey/packer,zhuzhih2017/packer,ricardclau/packer,tb3088/packer,hashicorp/packer,c22/packer,mkuzmin/packer,quantang/packer,paulmey/packer,grubernaut/packer,markpeek/packer,pieter-lazzaro/packer,timsutton/packer,grubernaut/packer,tb3088/packer,tsmolka/packer,quantang/packer,ChrisLundquist/packer,tb3088/packer,ChrisLundquist/packer,mkuzmin/packer,ricardclau/packer,routelastresort/packer,sean-/packer,p1-nico/packer,markpeek/packer,jen20/packer,bryson/packer,mitchellh/packer,c22/packer,mafrosis/packer,hashicorp/packer,zhuzhih2017/packer,bryson/packer,routelastresort/packer,rnaveiras/packer,arizvisa/packer,pieter-lazzaro/packer,markpeek/packer,youhong316/packer,atlassian/packer,tsmolka/packer,rnaveiras/packer,tas50/packer,pecigonzalo/packer,boumenot/packer,paulmey/packer,markpeek/packer,atlassian/packer,dayglojesus/packer,youhong316/packer,taliesins/packer,ricardclau/packer,tsmolka/packer,mafrosis/packer,grubernaut/packer,rnaveiras/packer,jen20/packer,routelastresort/packer,pieter-lazzaro/packer,pvandervelde/packer,mafrosis/packer,zhuzhih2017/packer,taliesins/packer,c22/packer,boumenot/packer,routelastresort/packer,smaato/packer,ChrisLundquist/packer,dave2/packer,grubernaut/packer,timsutton/packer,youhong316/packer,mkuzmin/packer,zhuzhih2017/packer,tas50/packer,sean-/packer,pvandervelde/packer,hashicorp/packer,paulmey/packer,zhuzhih2017/packer,grubernaut/packer,smaato/packer,KohlsTechnology/packer