Add point about unreductible match types raising error

Author: Kordyjan

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

@@ -78,6 +78,7 @@ that were true in Scala 2, will also remain true when compiled with Scala 3.
 - A `Wildcard` was made a subtype of `Ident` in the reflection API
 - `TypedOrTest` was added as a supertype of `Typed` in the reflection API
 - `Unapply.apply` was added to allow contruction of `Unapply` trees from macros
+- Unreductible match types are now raising type error
 - Scala 3.1 targets Scala.js 1.7.x+. This means that users must upgrade to Scala.js 1.7.0 or later to use Scala 3.1.
 
 Beside that Scala 3.1.0 introduced multiple small improvements and fixed a handful of bugs. You can see [the detailed changelog](https://github.com/lampepfl/dotty/releases/tag/3.1.0) on GitHub.