Add a paragraph about Mirrors for hierarchical sum types

Author: Kordyjan

blog/_posts/2021-10-19-scala-3.1.0-released.md

@@ -70,6 +70,20 @@ For the sake of compatibility with Scala 2, the Scala 3 compiler now generates g
 
 that were true in Scala 2, will also remain true when compiled with Scala 3.
 
+### Mirrors for Hierarchical Sealed Types
+
+Consider the following sealed hierarchy:
+
+```scala
+sealed trait Top
+case class Middle() extends Top with Bottom
+sealed trait Bottom extends Top
+```
+
+Previously you would not be able to summon an instance of `scala.deriving.Mirror.SumOf` for `Top`, as its child `Bottom` is also sealed. In Scala 3.1 this is now possible, enabling compiletime reflection over nested hierarchies of sealed types.
+
+The only change necessary for users is that you should now consider that one of the `MirroredElemTypes` for a `Mirror.SumOf could also be a sum type, not just product types.
+
 ### Other changes
 
 - Scastie was integrated into Scaladoc to make snippets interactive.