add explanation of ui message types
add explanation of ui message types

youhong316/packer,ricardclau/packer,tas50/packer,dave2/packer,ricardclau/packer,tb3088/packer,timsutton/packer,ricardclau/packer,tb3088/packer,grubernaut/packer,timsutton/packer,youhong316/packer,grubernaut/packer,paulmey/packer,mitchellh/packer,hashicorp/packer,ricardclau/packer,timsutton/packer,dave2/packer,pecigonzalo/packer,rnaveiras/packer,rnaveiras/packer,mitchellh/packer,mkuzmin/packer,mkuzmin/packer,pecigonzalo/packer,dave2/packer,dave2/packer,tas50/packer,mkuzmin/packer,youhong316/packer,ricardclau/packer,mitchellh/packer,tas50/packer,hashicorp/packer,rnaveiras/packer,tas50/packer,paulmey/packer,grubernaut/packer,bryson/packer,hashicorp/packer,paulmey/packer,rnaveiras/packer,tb3088/packer,mkuzmin/packer,pecigonzalo/packer,bryson/packer,paulmey/packer,youhong316/packer,mkuzmin/packer,rnaveiras/packer,pecigonzalo/packer,mitchellh/packer,bryson/packer,mkuzmin/packer,timsutton/packer,grubernaut/packer,ricardclau/packer,mitchellh/packer,paulmey/packer,timsutton/packer,tas50/packer,tb3088/packer,timsutton/packer,tb3088/packer,dave2/packer,youhong316/packer,grubernaut/packer,youhong316/packer,bryson/packer,rnaveiras/packer,bryson/packer,paulmey/packer,tb3088/packer,dave2/packer,bryson/packer,grubernaut/packer,hashicorp/packer,tas50/packer