Release Notes; upgrade information mainly.
Features/major bugfixes are listed in NEWS

portage-2.1
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* new cache framework, breaking all old cache modules.
  If you're having problems with portage_db_cdb, this is likely the cause.
* USE flag output ordering has changed.  The old ordering is now an option
  by the name of --alphabetical.  Adding the option to EMERGE_DEFAULT_OPTS
  in make.conf will restore the old behaviour permanently.