Document validate-borgmatic-config and add a few tests.

docs/how-to/set-up-backups.md

@@ -76,7 +76,21 @@ FAQ](http://borgbackup.readthedocs.io/en/stable/faq.html#how-can-i-specify-the-e
 for more info.
 
 
-###
+### Validation
+
+If you'd like to validate that your borgmatic configuration is valid, the
+following command is available for that:
+
+```bash
+sudo validate-borgmatic-config
+```
+
+This command's exit status (`$?` in Bash) is zero when configuration is valid
+and non-zero otherwise.
+
+Validating configuration can be useful if you generate your configuration
+files via configuration management, or you just want to double check that your
+hand edits are valid.
 
 
 ## Initialization