update README for XL test

Smilei/testcase_xlarge/README.md

@@ -11,7 +11,8 @@ It requires about 250GB to run.
 Prepare the test case
 ---------------------
 
-This test case restarts from a checkpoint. Make sure to run the prepare.sh script in your working directory before running the application.
+This test case restarts from a checkpoint. You must define the location of the checkpoint xlarge\_checkpoint.tar file in the prepare.sh script. 
+Make sure to run the prepare.sh script in your working directory before running the application.
 
 Case profile
 ------------

Smilei/testcase_xlarge/prepare.sh

 #!/bin/bash
-echo "No preparation needed"
+echo "Untar the checkpoint"
 
 tar -xvf /store/CINES/dci/SHARED/abs/Smilei/xlarge_checkpoint.tar