Update INSTALL.md

INSTALL.md

@@ -224,6 +224,7 @@ It is intended to provide a way to use various mail services in the future,
 just bear with us for the time being (or open a Merge Request).
 
 ```bash
+mkdir -p /srv/walnut/var/$my_domain
 vim /srv/walnut/var/$my_domain/config.json
 ```