Add support for SMTP

Signed-off-by: Shin'ya Minazuki <shinyoukai@laidback.moe>

README.md

@@ -220,6 +220,9 @@ Add the following to your `config.php`:
 **⚠⚠ Warning:** If you need to set *5 (Hashed Password in Database)* to false, your Prosody Instance is storing passwords in plaintext. This is insecure and not recommended. We highly recommend that you change your Prosody configuration to protect the passwords of your Prosody users. ⚠⚠
 
 
+## SMTP
+Works the same way as the IMAP section above, but authenticates against an SMTP server.
+
 Alternatives
 ------------
 Other extensions allow connecting to external user databases directly via SQL, which may be faster: