Add entry in README about Mailbox::addMessage (#325)

* Add entry in README about Mailbox::addMessage

* Use more comprehensive tools.ietf.org

Author: soywod

README.md

@@ -120,6 +120,15 @@ foreach ($messages as $message) {
 }
 ```
 
+To insert a new message (that just has been sent) into the Sent mailbox and flag it as seen:
+
+```php
+$mailbox = $connection->getMailbox('Sent');
+$mailbox->addMessage($messageMIME, '\\Seen');
+```
+
+Note that the message should be a string at MIME format (as described in the [RFC2045](https://tools.ietf.org/html/rfc2045)).
+
 #### Searching for Messages
 
 ```php