Add information about how to set permissions for postfix input (#3594)

(cherry picked from commit a440ed8d8c)

plugins/inputs/postfix/README.md

@@ -13,6 +13,25 @@ For each of the active, hold, incoming, maildrop, and deferred queues (http://ww
   # queue_directory = "/var/spool/postfix"
 ```
 
+#### Permissions:
+
+Telegraf will need read access to the files in the queue directory.  You may
+need to alter the permissions of these directories to provide access to the
+telegraf user.
+
+Unix permissions:
+```sh
+$ sudo chgrp -R telegraf /var/spool/postfix/{active,hold,incoming,deferred}
+$ sudo chmod -R g+rXs /var/spool/postfix/{active,hold,incoming,deferred}
+$ sudo usermod -a -G postdrop telegraf
+$ sudo chmod g+r /var/spool/postfix/maildrop
+```
+
+Posix ACL:
+```sh
+$ sudo setfacl -Rdm u:telegraf:rX /var/spool/postfix/{active,hold,incoming,deferred,maildrop}
+```
+
 ### Measurements & Fields:
 
 - postfix_queue