README: Update documentation wrt. Patch-mainline
    
    Common practice is to set Patch-mainline to a Linux release tag. More
    than 95% of all patches follow this convention. The remaining 5% have
    been fixed accordingly in SLE15.
    
    The README file is inconsistent wrt. to the content of Patch-mainline.
    In some places it refers to a release tag, in others it refers to a version
    number. With this cleanup, it refers to release tags everywhere.
    
    This change is a follow-up for commit 1d81d2699cd3.