Enhance README with spreadsheet usage instructions

Added sections for single and multiple subject spreadsheets with links to templates and examples.

Author: CPernet

spreadsheet_conversion/Readme.md

@@ -2,12 +2,16 @@
 
 When data are already converted, it can be easier to create metadata separately, here as json file to follow [PET-BIDS](https://bids-specification.readthedocs.io/en/stable/04-modality-specific-files/09-positron-emission-tomography.html).
 
-## Spreadsheet
+## 1 spreadsheet per subject
 
 You can use the [single-subject template](https://github.com/openneuropet/PET2BIDS/raw/refs/heads/main/spreadsheet_conversion/single_subject_sheet/subject_metadata_template.xlsx) to prepare the metadata. Note that new/old excel (.xlsx .xls) or open format (.ods) can be used.  For reference, here is an [example spreadsheet](https://github.com/openneuropet/PET2BIDS/raw/refs/heads/main/spreadsheet_conversion/single_subject_sheet/subject_metadata_example.xlsx)
 
 The example files, show the excel file and the resulting conversion as json.
 
+## 1 spreadsheet for many subjects
+
+In most cases, you subjects are converted (say using dcm2niix) and you need to update the json files with the metadata for all subjects. This is easily acheived using our preformated [scanner excel sheet](https://github.com/openneuropet/PET2BIDS/blob/main/spreadsheet_conversion/many_subjects_sheet/scanner_metadata_template.xlsx) which applies that information to all and the [tracer related excel sheet](https://github.com/openneuropet/PET2BIDS/blob/main/spreadsheet_conversion/many_subjects_sheet/subjects_metadata_template.xlsx). 
+
 ## convert_spreadsheet_metadata.m
 
-The matlab function `convert_spreadsheet_metadata.m` can be called and users are prompted to select a spreadsheet, or it can be passed directly as input. If command line is used, output name can also be specified, otherwise the same location and name is used to create the json file.
+The matlab function `convert_spreadsheet_metadata.m` can be called and users are prompted to select a spreadsheet, or it can be passed directly as input. If command line is used, output name can also be specified, otherwise the same location and name is used to create the json file.