add table of contents

cluster-trace-gpu-v2020/README.md

@@ -13,6 +13,20 @@ We provide the trace as is. We would encourage anybody who uses this trace to ci
 }
 ```
 
+**Table of Contents**
+
+- [1 Introduction](#1-introduction)
+- [2 Data Files for the Trace](#2-data-files-for-the-trace)
+  * [2.1 Overview](#21-overview)
+  * [2.2 Schema](#22-schema)
+    + [pai_job_table](#pai-job-table)
+    + [pai_task_table](#pai-task-table)
+    + [pai_instance_table](#pai-instance-table)
+    + [pai_sensor_table](#pai-sensor-table)
+    + [pai_group_tag_table](#pai-group-tag-table)
+    + [pai_machine_spec](#pai-machine-spec)
+    + [pai_machine_metric](#pai-machine-metric)
+- [3 Demo of Data Analysis](#3-demo-of-data-analysis)
 
 
 # 2 Data Files for the Trace