docs: update README

docs/using-the-mysql-or-mariadb-backend.md

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+# Using the MySQL(MariaDB) backend
+
+Now supports MySQL(MariaDB) backend.
+
+<br>
+
+
+
+## Environment Variables
+
+#### DB_TYPE
+
+Set to `mysql` switch to MySQL(MariaDB) database.
+
+Default: `sqlite`
+
+#### MYSQL_HOST
+
+MySQL(MariaDB) host, **required**.
+
+#### MYSQL_PORT
+
+MySQL(MariaDB) port.
+
+Default: `3306`
+
+#### MYSQL_DATABASE
+
+MySQL(MariaDB) database name.
+
+Default: `vaultwarden`
+
+#### MYSQL_USERNAME
+
+MySQL(MariaDB) username.
+
+Default: `vaultwarden`
+
+#### MYSQL_PASSWORD
+
+MySQL(MariaDB) password, **required**.
+
+<br>
+
+
+
+## Backup
+
+Specify the above environment variables to switch to the MySQL(MariaDB) database.
+
+<br>
+
+
+
+## Restore
+
+When restoring, also specify the above environment variables to switch to the MySQL(MariaDB) database.
+
+1. Ensure that the database is accessible.
+
+Perhaps you will use the `docker-compose up -d [services name]` command to start the database separately.
+
+2. Verify that the `MYSQL_HOST` you are using is accessible to.
+
+If your database is running in docker-compose, you need to find the corresponding network name via `docker network ls`  and add `--network=[name]` to the restore command to specify the network name.
+
+3. Restore and restart the container.