document proxyprotocol

ChangeLog

@@ -5,6 +5,8 @@ vNEXT:
 	(listening or connecting) will be performed on Unix
 	socket instead of Internet sockets.
 
+	Support proxyprotocol on the backend server side.
+
 v2.1.3:
 	Fix Landlock access to /etc/hosts.deny and
 	/etc/hosts.allow.