[HowTo] Replace courier with dovecot

[aseques]

I've just added details about how to add sieve rules to the system. It should work without problems together with dovecot.

With these rules, if you have a antispam before the server adding the tag X-Spam-Flag=yes to the headers, then dovecot will automatically move the mail to the Junk folder.
If that's not the case (i.e. you don't have antispam), mail will work as usual.