README: Update the Before You Commit section (jsc#PED-5021)
    
    * Prefix the example invocation of scripts/sequence-patch.sh with "./"
      for consistency with the rest of the document.
    * Update the example output from scripts/sequence-patch.sh to match the
      regular invocation instead of the Rapidquilt case and bump the output
      to SLE15-SP5.
    * Drop the paragraph describing that a fix patch should be placed in
      series.conf close to the patch which introduced the associated bug.
      The current situation is that the patches should be sorted according
      to the upstream order.
    * Add a new paragraph describing use of scripts/sequence-patch.sh with
      Rapidquilt.
    * Fix typos, slightly improve wording and integrate some occurrences of
      additional details in parentheses.