Add postscreen whitelist syntax and examples

docs/u_e-postfix-postscreen_whitelist.md

@@ -1,3 +1,20 @@
 IPs can be removed from Postscreen and therefore _also_ from RBL checks in `data/conf/postfix/custom_postscreen_whitelist.cidr`.
 
 Postscreen does multiple checks to identify malicious senders. In most cases you want to whitelist an IP to exclude it from blacklist lookups.
+
+The format of the file is as follows:
+
+`CIDR   ACTION`
+
+Where CIDR is a single IP address or IP range in CIDR notation, and action is either "permit" or "reject".
+
+Example:
+
+```
+# Rules are evaluated in the order as specified.
+# Blacklist 192.168.* except 192.168.0.1.
+192.168.0.1          permit
+192.168.0.0/16       reject
+```
+
+The file is reloaded on the fly, postfix restart is not required.