adjust readme for new plugin

mosquitto.conf-sample

@@ -0,0 +1,21 @@
+log_dest stdout
+
+persistence false
+
+listener 1883
+protocol mqtt
+#allow_anonymous true
+allow_anonymous false
+
+auth_plugin /opt/lib/go-auth.so
+auth_opt_log_dest stdout
+auth_opt_log_level debug
+auth_opt_backends mysql
+auth_opt_mysql_host mariadb
+auth_opt_mysql_port 3306
+auth_opt_mysql_dbname mosquittoauth
+auth_opt_mysql_user mosquittoauth
+auth_opt_mysql_password xxx
+auth_opt_mysql_allow_native_passwords true
+auth_opt_mysql_userquery SELECT pw FROM users WHERE username = ?
+auth_opt_mysql_aclquery SELECT topic FROM acls WHERE username = ? AND (rw & ?) != 0