Add OpenTofu example (#2)

* Add readme

* Add OpenTofu example

---------

Co-authored-by: Fedor Batonogov <f.batonogov@yandex.ru>

opentofu/test_vm/variables.tf

@@ -0,0 +1,9 @@
+variable "virtual_environment_endpoint" {
+  type        = string
+  description = "The endpoint for the Proxmox Virtual Environment API (example: https://host:port)"
+}
+
+variable "virtual_environment_api_token" {
+  type        = string
+  description = "The api roken the Proxmox Virtual Environment API (example: root@pam!for-terraform-provider=xxxxxxxx-xxxx-xxxx-xxxx-xxxxxxxxxxxx)"
+}