Add documentation on how to make backups redundant with multiple repositories.

docs/how-to/make-backups-redundant.md

@@ -0,0 +1,47 @@
+---
+title: How to make backups redundant
+---
+## Multiple repositories
+
+If you really care about your data, you probably want more than one backup of
+it. borgmatic supports this in its configuration by specifying multiple backup
+repositories. Here's an example:
+
+```yaml
+location:
+    # List of source directories to backup.
+    source_directories:
+        - /home
+        - /etc
+
+    # Paths of local or remote repositories to backup to.
+    repositories:
+        - 1234@usw-s001.rsync.net:backups.borg
+        - k8pDxu32@k8pDxu32.repo.borgbase.com:repo
+        - /var/lib/backups/local.borg
+```
+
+When you run borgmatic with this configuration, it invokes Borg once for each
+configured repository in sequence. (So, not in parallel.) That means—in each
+repository—borgmatic creates a single new backup archive containing all of
+your source directories.
+
+Here's a way of visualizing what borgmatic does with the above configuration:
+
+1. Backup `/home` and `/etc` to `1234@usw-s001.rsync.net:backups.borg`
+2. Backup `/home` and `/etc` to `k8pDxu32@k8pDxu32.repo.borgbase.com:repo`
+3. Backup `/home` and `/etc` to `/var/lib/backups/local.borg`
+
+This gives you redundancy of your data across repositories and even
+potentially across providers.
+
+See [Borg repository URLs
+documentation](https://borgbackup.readthedocs.io/en/stable/usage/general.html#repository-urls)
+for more information on how to specify local and remote repository paths.
+
+
+## Related documentation
+
+ * [Set up backups with borgmatic](https://torsion.org/borgmatic/docs/how-to/set-up-backups/)
+ * [Make per-application backups](https://torsion.org/borgmatic/docs/how-to/make-per-application-backups/)
+ * [Backup to a removable drive or an intermittent server](https://torsion.org/borgmatic/docs/how-to/backup-to-a-removable-drive-or-an-intermittent-server/)