Document how to add marketplace agreement through Terraform

[kellystuard]

If using Terraform to accept the terms and conditions, the module needs an explicit dependency to ensure the resources are created in the proper order:

resource "azurerm_marketplace_agreement" "spacelift_worker" {
  publisher = "spaceliftinc1625499025476"
  offer     = "spacelift_worker"
  plan      = "ubuntu_20_04"
}

module "azure-worker" {
  source     = "github.com/spacelift-io/terraform-azure-spacelift-workerpool?ref=v0.1.0"
  depends_on = [ azurerm_marketplace_agreement.spacelift_worker ]

  [...]
}

More information can be found here.