Upgrading Guide (#239)

Update docs to upgrade to new major v4.*.* version.

website/docs/guides/upgrading_to_version_4.html.markdown

@@ -0,0 +1,40 @@
+---
+layout: "selectel"
+page_title: "Upgrading Terraform Selectel Provider to version 4.0.0"
+sidebar_current: "docs-selectel-guide-upgrade-guide"
+description: |-
+  How to upgrade Terraform Selectel Provider to version 4.0.0.
+---
+
+# Upgrading Terraform Selectel Provider to version 4.0.0 
+
+To upgrade Terraform Selectel Provider version to the new major version 4.0.0:
+
+1. In the [Control Panel](https://my.selectel.ru/profile/users_management/users?type=service), create a service user with an Account Administrator role. Learn more [how to create a service user](https://docs.selectel.ru/control-panel-actions/users-and-roles/add-user/).
+2. Change the authorization block.
+
+    From:
+
+    ```hcl
+    provider "selectel" {
+      token = <token>
+    }
+    ```
+
+    To: 
+
+    ```hcl
+    provider "selectel" {
+      domain_name = <account_id>
+      username    = <username>
+      password    = <password>
+    }
+    ```
+
+    where:
+
+    * `<account_id>` - (Required) Selectel account ID. The account ID is in the top right corner of the [Control panel](https://my.selectel.ru/). For import, use the value in the `OS_DOMAIN_NAME` environment variable. Learn more about [Registration](https://docs.selectel.ru/control-panel-actions/account/registration/).
+
+    * `<username>` - (Required) Name of the service user. To get the name, in the top right corner of the [Control panel](https://my.selectel.ru/profile/users_management/users?type=service), go to the account menu ⟶ **Profile and Settings** ⟶ **User management** ⟶ the **Service users** tab ⟶ copy the name of the required user. For import, use the value in the `OS_USERNAME` environment variable. Learn more about [Service users](https://docs.selectel.ru/control-panel-actions/users-and-roles/user-types-and-roles/) and [how to create service user](https://docs.selectel.ru/control-panel-actions/users-and-roles/add-user/#добавить-сервисного-пользователя).
+
+    * `<password>` - (Required, Sensitive) Password of the service user. For import, use the value in the `OS_PASSWORD` environment variable.