Update README.md

update duration of dataset.

* container: about 12 hours
* server and batch: about 24 hours

README.md

@@ -1,5 +1,5 @@
 # Overview
-The trace data, ClusterData201708,  contains cluster information of a production cluster in 12 hours period, and contains about 1.3k machines that run both online service and batch jobs.
+The trace data, ClusterData201708,  contains cluster information of a production cluster in 12 hours period (see note below), and contains about 1.3k machines that run both online service and batch jobs.
 
 The data is provided to address [the challenges Alibaba face](https://github.com/alibaba/clusterdata/wiki/About-Alibaba-cluster-and-why-we-open-the-data) in idcs where online services and batch jobs are co-allocated.  We distill the challenges as the following topics: 
 
@@ -9,6 +9,8 @@ The data is provided to address [the challenges Alibaba face](https://github.com
 
 Please let us know if you have any issues, ideas, or papers about these data by sending email to us [aliababa-clusterdata](mailto:alibaba-clusterdata@list.alibaba-inc.com). The more specific the feedback, the more likely we are to be able to help you.
 
+*note for 12 hours period:  although the data for server and batch spans about 24hours, data for containers is refined to 12 hours. We will release another version in near future.*
+
 # Trace data 
 The format of trace data is described in the [schema description](trace_201708.md), and defined in the specification file [schema.csv](schema.csv) in the repository.