Update README.md

For the clarity of evaluation pipeline

Author: Mrmoore98

README.md

@@ -34,7 +34,12 @@ Run `python vis_label.py ` for demo of vectorized labels. The visualizations are
 Run `python train.py --instance_seg --direction_pred --version [v1.0-trainval, v1.0-mini] --logdir [output place]`. 
 
 ### Evaluation
-Run `python evaluate.py --result_path [submission file]` for evaluation. The script accepts vectorized or rasterized maps as input. For vectorized map, We firstly rasterize the vectors to map to do evaluation. For rasterized map, you should make sure the line width=1.
+Before running the evaluation code, you should get the `submission.json` file first, which can be generated by the following command.
+```
+python export_gt_to_json.py
+```
+
+Run `python evaluate.py --result_path [submission file path]` for evaluation. The script accepts vectorized or rasterized maps as input. For vectorized map, We firstly rasterize the vectors to map to do evaluation. For rasterized map, you should make sure the line width=1.
 
 Below is the format for vectorized submission: