2.12.13 release notes fixup

Author: SethTisue

_posts/2021-01-12-release-notes-2.12.13.md

@@ -5,4 +5,6 @@ title: "Scala 2.12.13 is now available!"
 ---
 [Scala 2.12.13](https://github.com/scala/scala/releases/tag/v2.12.13) is now available!
 
+This release backports configurable warnings (`-Wconf` and `@nowarn`) from 2.13. It also includes a few other changes an fixes.
+
 For all the details, refer to the [release notes](https://github.com/scala/scala/releases/tag/v2.12.13) on GitHub.