add road-network datasets (including the selected five cities)

README.md

@@ -36,10 +36,14 @@ The structure of "./data/raw/" should be like:
         * ...    
     * pickup  
         * pickup_sh.csv  
-        * ...  
+        * ...
+    * road-network  
+        * roads.csv  
 ```
 
-Each sub-dataset contains 5 csv files, with each representing the data from a specific city,  the detail of each city can be find in the following table.
+road-network/roads.csv records the road network of the five cities.
+
+Each sub-dataset (delivery, pickup) contains 5 CSV files, with each representing the data from a specific city,  the detail of each city can be find in the following table.
 
 
 |   City     |  Description                                                                                  |