HDFS-5698. Update CHANGES.txt in branch-2

git-svn-id: https://svn.apache.org/repos/asf/hadoop/common/branches/branch-2@1568584 13f79535-47bb-0310-9956-ffa450edef68

hadoop-hdfs-project/hadoop-hdfs/CHANGES.txt

@@ -6,6 +6,9 @@ Release 2.4.0 - UNRELEASED
 
   NEW FEATURES
 
+    HDFS-5698. Use protobuf to serialize / deserialize FSImage. (See breakdown
+    of tasks below for features and contributors)
+
   IMPROVEMENTS
 
     HDFS-5781. Use an array to record the mapping between FSEditLogOpCode and