MXS-872: Document new grant requirement

The user now requires SELECT privileges on the mysql.roles_mapping
table.

Currently this is a mandatory grant but it needs to be made into an
optional requirement. This allows upgrades from 2.2.9 to 2.2.10 without
needing new grants.

Documentation/Tutorials/MaxScale-Tutorial.md

@@ -32,6 +32,7 @@ CREATE USER 'maxscale'@'%' IDENTIFIED BY 'maxscale_pw';
 GRANT SELECT ON mysql.user TO 'maxscale'@'%';
 GRANT SELECT ON mysql.db TO 'maxscale'@'%';
 GRANT SELECT ON mysql.tables_priv TO 'maxscale'@'%';
+GRANT SELECT ON mysql.roles_mapping TO 'maxscale'@'%';
 GRANT SHOW DATABASES ON *.* TO 'maxscale'@'%';
 ```