mention package for Mirror
mention package for Mirror

Newcomers (like me) don't know where `Mirror` comes from and there's actually a lot of Scala 3 content that skips over mentioning this
sjrd/dotty,sjrd/dotty,lampepfl/dotty,dotty-staging/dotty,sjrd/dotty,sjrd/dotty,lampepfl/dotty,dotty-staging/dotty,lampepfl/dotty,sjrd/dotty,dotty-staging/dotty,lampepfl/dotty,dotty-staging/dotty,lampepfl/dotty,dotty-staging/dotty