Add secure connection TLS/SSL for the SMTP transport

docs/email-configuration.markdown

@@ -34,6 +34,24 @@ define('MAIL_SMTP_USERNAME', 'username');
 define('MAIL_SMTP_PASSWORD', 'super password');
 ```
 
+It's also possible to use a secure connection, TLS or SSL:
+
+```php
+define('MAIL_SMTP_ENCRYPTION', 'ssl'); // Valid values are "null", "ssl" or "tls"
+```
+
+Here an example with Google:
+
+```php
+define('MAIL_SMTP_HOSTNAME', 'smtp.gmail.com');
+define('MAIL_SMTP_PORT', 465);
+define('MAIL_SMTP_USERNAME', 'my_account@gmail.com');
+define('MAIL_SMTP_PASSWORD', 'my google password');
+define('MAIL_SMTP_ENCRYPTION', 'ssl');
+```
+
+To use Google, you might need to allow Kanboard to use your Google account, see ["Allowing less secure apps to access your account"](https://support.google.com/accounts/answer/6010255) and ["My client isn't accepting my username and password"](https://support.google.com/mail/answer/14257).
+
 ### Sendmail configuration
 
 By default the sendmail command will be `/usr/sbin/sendmail -bs` but you can customize that in your config file.