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

portage-2.1
==================================

* 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.
* The deprecated --inject has been removed, use /etc/portage/profile/package.provided
* The deprecated --upgradeonly has been removed, use /etc/portage/package.* 
  instead.
* 'emerge sync' has been deprecated, use 'emerge --sync' instead (same 
  for other actions)
* Tools that call emerge should override the EMERGE_DEFAULT_OPTS environment
  variable or use the emerge --ignore-default-opts option.
* rsync option handling has been redesigned, instead of RSYNC_* variables
  use PORTAGE_RSYNC_EXTRA_OPTS from now on.