docs: update README

docs/manually-trigger-a-backup.md

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+# Manually trigger a backup
+
+Sometimes, it's necessary to manually trigger backup actions.
+
+This can be useful when other programs are used to consistently schedule tasks or to verify that environment variables are properly configured.
+
+<br>
+
+
+
+## Usage
+
+Previously, performing an immediate backup required overwriting the entrypoint of the image. However, with the new setup, you can perform a backup directly with a parameterless command.
+
+```shell
+docker run \
+  --rm \
+  --name vaultwarden_backup \
+  --volumes-from=vaultwarden \
+  --mount type=volume,source=vaultwarden-rclone-data,target=/config/ \
+  -e ... \
+  ttionya/vaultwarden-backup:latest backup
+```
+
+You also need to mount the rclone config file and set the environment variables.
+
+The only difference is that the environment variable `CRON` does not work because it does not start the CRON program, but exits the container after the backup is done.
+
+<br>
+
+
+
+## IMPORTANT
+
+**Manually triggering a backup only verifies that the environment variables are configured correctly, not that CRON is working properly. This is the [issue](https://github.com/ttionya/vaultwarden-backup/issues/53) that CRON may not work properly on ARM devices.**
+