[Dovecot] How to add dovecot plugin?

[Timothy Murphy]

I'm running dovecot version 2.0.9 under CentOS-6.5 .
I want to add the following plugin:
====================
require "fileinto";
if header :contains "X-Spam-Flag" "YES" {
  fileinto "Spam";
}
====================
Could some kind soul tell me where exactly I should place this script
(eg "anywhere in /etc/dovecot/dovecot.conf"
or "anywhere in /etc/dovecot/conf.d/90-plugin.conf"),
and do I need to take any other action?

Actually, the folder I should like spam to go to is
~/Maildir/.Spam/cur/ - should I modify the above script?

If these queries are anwered clearly in some dovecot documentation,
please point me to that.

-- 
Timothy Murphy  
e-mail: gayleard /at/ eircom.net
School of Mathematics, Trinity College, Dublin 2, Ireland