README: Update the Embargoed Patches section (jsc#PED-5021)
    
    * Improve wording and style: avoid use of the "e.g." and "i.e."
      abbreviations, etc.
    * Update the example branch names to SLE15-SP5.
    * Remove the example how to merge the embargoed branch back because the
      commands should be obvious to anyone dealing with embargoed branches.