Document how to migrate the provider from <0.13 to >0.13

[camrossi]

Currently terrafromer generate a config that is not compatible with the new version of TF.

To migrate to Terrafrom > 0.13 the following steps are required

1. Replace the provider

terraform state replace-provider -auto-approve "registry.terraform.io/-/aci" "registry.terraform.io/ciscodevnet/aci"
2. Edit the provider.tf and replace

terraform {
	required_providers {
		aci = {
	    version = "~> 2.3.0"
		}
  }
}

with

terraform {
  required_providers {
    aci = {
      source = "CiscoDevNet/aci"
      version = "2.3.0"
    }
  }
}