Update Readme.md

Update description of longitudinal part of shanoir2bids

Readme.md

@@ -52,10 +52,12 @@ The dictionary in the json file must have four keys :
     -> "bidsDir"    : str, BIDS subdirectory sequence name (eg : "anat", "func" or "dwi", ...)
     -> "bidsName"   : str, BIDS sequence name (eg: "t1w-mprage", "t2-hr", "cusp66-ap-b0", ...)
 ```
-To download longitudinal data, a key `session` and a new entry `bidsSession` in `data_to_bids` dictionaries should be defined in the JSON configuration files. 
 
 An example is provided in the file `s2b_example_config.json`.
 
+To download longitudinal data, a key `session` and a new entry `bidsSession` in `data_to_bids` dictionaries should be defined in the JSON configuration files. Of note, only one session can be downloaded at once. Then, the key `session` is just a string, not a list as for subjects.
+
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 ### Example usage
 
 To download datasets, verify the content of them, anonymize them and / or encrypt them you can use a command like: