Add URL-encoding notice

I've had trouble connecting to Amazon SES via recommended method.
I kept getting: 
> Exception occurred while flushing email queue: Failed to authenticate on SMTP server with username "***"
I've traced it to "+" sign in the password that Amazon SES automatically assigned to my SMTP credentials.
I had a hunch that special characters might be a problem and was thinking along the lines of wrapping the whole MAILER_URL value in single quotes.
Then I ran into https://stackoverflow.com/a/50498999/1419874 that solved the issue for me too.
Since the OP lost 3hrs and myself almost an hour, I think URL encoding might not be so obvious.
Hence I propose this change. I've intentionally put it at the top, since I think it applies to any method that has special characters in username/password, not just Amazon SES.

Author: Nebojša Kamber

email.rst

@@ -39,6 +39,7 @@ environment variable in the ``.env`` file:
 
     # use this to configure a traditional SMTP server
     MAILER_URL=smtp://localhost:25?encryption=ssl&auth_mode=login&username=&password=
+    # keep in mind that, since this is an URL, any special characters in username/password should be URL-encoded
 
 Refer to the :doc:`SwiftMailer configuration reference </reference/configuration/swiftmailer>`
 for the detailed explanation of all the available config options.