Added file missing from previous commit

+package org.onlab.netty;
+
+/**
+ * State transitions a decoder goes through as it is decoding an incoming message.
+ */
+public enum DecoderState {
+    READ_HEADER_VERSION,
+    READ_PREAMBLE,
+    READ_CONTENT_LENGTH,
+    READ_SERIALIZER_VERSION,
+    READ_CONTENT
+}