[DOC] Add the readme to get started

refs KIME-3340

README.md

+This Flow package extends the [Neos Swift Mailer](https://github.com/neos/swiftmailer) by a spool for asynchronous mailing.
+
+## Configuration
+
+The package is pre-configured to use a the file spool:
+
+	WE:
+	  SwiftMailerSpool:
+		spool:
+		  type: 'Swift_FileSpool'
+		  options: []
+		  arguments:
+			path: %FLOW_PATH_DATA%/SwiftMailerSpool/
+			
+## Usage
+
+Different from the [Neos Swift Mailer](https://github.com/neos/swiftmailer), a `\Swift_Message` has to be created instead 
+of using the `\TYPO3\SwiftMailer\Message` object. This is due to a problems on the serialization of the message that 
+happens in the `\FileSpool` as the serialized `\TYPO3\SwiftMailer\Message` does not include its parent properties of the 
+`\Swift_Message`. 
+
+The process is the same as with the normal SwiftMailer library.
+
+Inject the mailer interface which now is a `\Swift_SpoolTransport` object:
+
+	/**
+	 * @Flow\Inject
+	 * @var \TYPO3\SwiftMailer\MailerInterface
+	 */
+	protected $mailer;
+
+Create the message:
+
+	$mail = new \Swift_Message();
+
+Send the message with the mailer:
+
+	$this->mailer->send($mail);
+
+Now, the mail is in the spool and can really be sent by the command:
+
+	./flow swiftmailerspool:flush
+