Consider documenting how to setup custom domains

[springcomp]

See https://github.com/springcomp/self-hosted-simplelogin/issues/7#issuecomment-1844211594.

When adding a custom domain, say my-custom-domain.tld the Postfix configuration most likely needs to be adjusted. The domain must be included as one of the virtual_alias_domains entries like so:

  maillog_file=/dev/stdout

- virtual_alias_domains = 
+ virtual_alias_domains = my-custom-domain.tld
  virtual_alias_maps = lmdb:/etc/postfix/conf.d/virtual, regexp:/etc/postfix/conf.d/virtual-regexp

TODO:

• [ ] Test the scenario
• [ ] Document required changes

[springcomp]

In fact, subdomains work out of the box.