SI-6566: Announce fix in release notes

Because of SI-6566, right-hand sides of type aliases are now considered invariant for variance checking. Mention this in the release notes, per discussion with @retronym [on JIRA](https://issues.scala-lang.org/browse/SI-6566?#comment-68243).
Merge pull request #46 from Blaisorblade/patch-1

SI-6566: Announce fix in release notes