Update release notes wrt #1085

Author: k163377

release-notes/CREDITS-2.x

@@ -18,6 +18,7 @@ Contributors:
 # 2.21.0 (not yet released)
 
 WrongWrong (@k163377)
+* #1085: Added addMixIn function for generic MapperBuilder
 * #1043: Cleanup of deprecated contents
 * #1042: Remove old StrictNullChecks
 * #1041: Remove MissingKotlinParameterException

release-notes/VERSION-2.x

@@ -18,6 +18,7 @@ Co-maintainers:
 
 2.21.0 (not yet released)
 
+#1085: The addMixIn extension function is now available in the generic MapperBuilder.
 #1043: Deprecated content has been cleaned up with the version upgrade.
 #1042: The old StrictNullChecks backend has been removed.
   This improves the throughput of deserialization slightly.