Update README.md with spam prevention

[edgar-koster]

I've added a section on how you can prevent mail to be treated as spam. This can be quite common when you change the from address to that of a user email address. In fact, it usually will occur if the from address domain is not linked to the sending host

I've spent quit some time to get this working in my old application where I used my own node-mailer solution (but with featherjs backend). That application is a multi-tenancy app, and customers can choose to use DKIM or not. If they do, the information can be entered on a settings form. Each time a mail is sent, the information is collected.