add data_with_trajectory_20s usage

README.md

@@ -44,6 +44,8 @@ The structure of "./data/raw/" should be like:
 ```
 
 road-network/roads.csv records the road network of the five cities.
+
+
 data_with_trajectory_20s/* records the trajectory of courier every 20 seconds.
 ```python
 import pandas as pd