Update README.md

[Irytacja]

I've added undocumented config option.

[vlizanae]

Since this is open maybe you could fix a typo in the readme? in mail_options the example

"mail_options": {
	"secure", true,
	"auth": { user: "fsmith", pass: "12345" },
	"connectionTimeout": 10000,
	"greetingTimeout": 10000,
	"socketTimeout": 10000
}

should actually be

"mail_options": {
	"secure": true,
	"auth": { "user": "fsmith", "pass": "12345" },
	"connectionTimeout": 10000,
	"greetingTimeout": 10000,
	"socketTimeout": 10000
}