README: Update the Kernel ABI Changes section (jsc#PED-5021)
    
    * Add a short description about stable kABI to give readers more
      context.
    * Rework the main part of the section to reflect that the ABI reference
      is stored in symvers and symtypes files, applies to SLE12 onwards.
    * Adjust the update-symvers example to note that in order to update both
      reference files, one has to pass to the script the default and devel
      packages for a respective kernel.
    * Drop the second update-symvers example which mentions use of --filter
      because the option should not be generally very useful to most people.
    * Update the note about who should update the kabi files to say that it
      should be branch maintainers.