Update release notes

Author: cowtowncoder

release-notes/CREDITS-2.x

@@ -329,7 +329,9 @@ Yoann Vernageau (@yvrng)
   (2.17.1)
 
 PJ Fanning (@pjfanning)
- * Contributed #484: Rework synchronization in `ProtobufMapper`
+ * Contributed #484: (protobuf) Rework synchronization in `ProtobufMapper`
+  (2.18.0)
+ * Contributed #508: (avro) Ignore `specificData` field on serialization
   (2.18.0)
 
 Joachim Lous (@jlous)

release-notes/VERSION-2.x

@@ -18,10 +18,12 @@ Active maintainers:
 
 #484: (protobuf) Rework synchronization in `ProtobufMapper`
  (contributed by @pjfanning)
-#494: Avro Schema generation: allow mapping Java Enum properties to
+#494: (avro) Avro Schema generation: allow mapping Java Enum properties to
   Avro String values
  (requested by Joachim L)
  (contributed by Michal F)
+#508: (avro) Ignore `specificData` field on serialization
+ (contributed by @pjfanning)
 
 2.17.3 (not yet released)