Update README.md with visual check info

ASL_pipeline/README.md

 # ASL analysis pipeline
 Order of scripts:
 1) `bb_asl.sh`: ASL preprocessing using oxasl with structural and distortion correction
-2) `bb_asl_summary_measures.sh`: extracts summary CBF and ATT metrics in GM and WM
-3) `bb_IDP_asl.sh`: Script to generate the IDPs related to ASL and save them in a similar format to the other UK Biobank IDPs
+2) `bb_asl_visual_check.sh` : Script to visually inspect the output of the ASL processing pipeline
+3) `bb_asl_summary_measures.sh`: extracts summary CBF and ATT metrics in GM and WM
+4) `bb_IDP_asl.sh`: Script to generate the IDPs related to ASL and save them in a similar format to the other UK Biobank IDPs