Update Postfix configuration instructions 

Fixes #1139.
```sh
# cat /var/log/mail/mail.log
Aug  7 14:34:45 debian-607-x64-vbox4210-nocm postfix/postfix-script[15186]: refreshing the Postfix mail system
Aug  7 14:34:45 debian-607-x64-vbox4210-nocm postfix/master[14952]: reload -- version 2.7.1, configuration /etc/postfix
Aug  7 14:34:53 debian-607-x64-vbox4210-nocm postfix/pickup[15190]: ED7CF77E75: uid=0 from=<root>
Aug  7 14:34:53 debian-607-x64-vbox4210-nocm postfix/cleanup[15194]: ED7CF77E75: message-id=<20140807143453.ED7CF77E75@debian-607-x64-vbox4210-nocm.vm>
Aug  7 14:34:53 debian-607-x64-vbox4210-nocm postfix/qmgr[15191]: ED7CF77E75: from=<root@debian-607-x64-vbox4210-nocm.vm>, size=338, nrcpt=1 (queue active)
Aug  7 14:34:53 debian-607-x64-vbox4210-nocm postfix/pipe[15196]: ED7CF77E75: to=<foi+request-1234@example.com>, relay=alaveteli, delay=0.02, delays=0.01/0/0/0.01, dsn=2.0.0, status=deliverable (delivers to command: /var/www/alaveteli/script/mailin)
Aug  7 14:34:53 debian-607-x64-vbox4210-nocm postfix/cleanup[15194]: F075A77E78: message-id=<20140807143453.F075A77E78@debian-607-x64-vbox4210-nocm.vm>
Aug  7 14:34:53 debian-607-x64-vbox4210-nocm postfix/bounce[15197]: ED7CF77E75: sender delivery status notification: F075A77E78
Aug  7 14:34:53 debian-607-x64-vbox4210-nocm postfix/qmgr[15191]: ED7CF77E75: removed
Aug  7 14:34:53 debian-607-x64-vbox4210-nocm postfix/qmgr[15191]: F075A77E78: from=<>, size=2156, nrcpt=1 (queue active)
Aug  7 14:34:55 debian-607-x64-vbox4210-nocm postfix/local[15199]: F075A77E78: to=<vagrant@debian-607-x64-vbox4210-nocm.vm>, orig_to=<root@debian-607-x64-vbox4210-nocm.vm>, relay=local, delay=1, delays=0/0.01/0/1, dsn=2.0.0, status=sent (delivered to command: procmail -a "$EXTENSION")
Aug  7 14:34:55 debian-607-x64-vbox4210-nocm postfix/qmgr[15191]: F075A77E78: removed
Aug  7 14:36:13 debian-607-x64-vbox4210-nocm postfix/pickup[15190]: 504C377EBE: uid=0 from=<root>
Aug  7 14:36:13 debian-607-x64-vbox4210-nocm postfix/cleanup[15194]: 504C377EBE: message-id=<20140807143613.504C377EBE@debian-607-x64-vbox4210-nocm.vm>
Aug  7 14:36:13 debian-607-x64-vbox4210-nocm postfix/qmgr[15191]: 504C377EBE: from=<root@debian-607-x64-vbox4210-nocm.vm>, size=338, nrcpt=1 (queue active)
Aug  7 14:36:13 debian-607-x64-vbox4210-nocm postfix/pipe[15196]: 504C377EBE: to=<foi+request-1234@example.com>, relay=alaveteli, delay=0.01, delays=0.01/0/0/0, dsn=2.0.0, status=deliverable (delivers to command: /var/www/alaveteli/script/mailin)
Aug  7 14:36:13 debian-607-x64-vbox4210-nocm postfix/cleanup[15194]: 5110877ECE: message-id=<20140807143613.5110877ECE@debian-607-x64-vbox4210-nocm.vm>
Aug  7 14:36:13 debian-607-x64-vbox4210-nocm postfix/bounce[15198]: 504C377EBE: sender delivery status notification: 5110877ECE
Aug  7 14:36:13 debian-607-x64-vbox4210-nocm postfix/qmgr[15191]: 504C377EBE: removed
Aug  7 14:36:13 debian-607-x64-vbox4210-nocm postfix/qmgr[15191]: 5110877ECE: from=<>, size=2156, nrcpt=1 (queue active)
Aug  7 14:36:13 debian-607-x64-vbox4210-nocm postfix/local[15199]: 5110877ECE: to=<vagrant@debian-607-x64-vbox4210-nocm.vm>, orig_to=<root@debian-607-x64-vbox4210-nocm.vm>, relay=local, delay=0.01, delays=0/0/0/0, dsn=2.0.0, status=sent (delivered to command: procmail -a "$EXTENSION")
Aug  7 14:36:13 debian-607-x64-vbox4210-nocm postfix/qmgr[15191]: 5110877ECE: removed
```