LP #1531896: Add required MySQL privileges to pt-online-schema-change documentation

**Reported in Launchpad by nulll last update 02-02-2016 13:16:54

Running pt-online-schema-change fails due to missing privileges,
by now I found that the needed privileges are SUPER (for creating a trigger)
PROCESS (to check slaves)

but maybe more are needed
http://ospi.netcode.fi/blog/privileges-for-percona-online-schema-change.html

It woul be great if you could specify in the official documentation the needed privileges
https://www.percona.com/doc/percona-toolkit/2.2/pt-online-schema-change.html

Thanks!