Add hint about autoyast2 to AutoYaST export readme

The kernel parameter autoyast2 makes it easier for the user to activate
this function but is only supported by SLES12 and 13.2 so I have added
it additionally to the original command line.
Merge pull request #488 from SUSE/optimize_autoyast_readme_for_sles12

Add hint about autoyast2 to AutoYaST export readme