[DOC] Add documentation to retrieve the labels of connectivity matric…

…es (aramis-lab#1248)

docs/Pipelines/DWI_Connectome.md

@@ -51,6 +51,15 @@ The main output files are:
     This atlas is a subdivision of the cerebral cortex into gyri and sulci, and contains 74 regions per hemisphere.
     It was built on anatomical MRI of 24 healthy subjects from which 74 cortical ROIs were manually identified in each of the individual hemispheres.
 
+The matrices are encoded as [TSV](../glossary.md#tsv) files without labels for rows and columns.
+These labels are generated by [MRtrix3](../Third-party.md#mrtrix3) and the lookup tables should have been automatically downloaded by Clinica when the `dwi-connectome` pipeline has been run.
+More precisely, there are two lookup tables for extracting the relevant grey matter parcellations:
+
+- one to extract them from the default FreeSurfer segmentation (desikan_killiany), which should be in `<your_clinica_folder>/clinica/resources/mappings/fs_default.txt`
+- and one to extract them from the a2009s FreeSurfer, which should be in `<your_clinica_folder>/clinica/resources/mappings/fs_a2009s.txt`
+
+If you cannot find these files, you can copy-paste them from [here](https://raw.githubusercontent.com/MRtrix3/mrtrix3/master/share/mrtrix3/labelconvert/fs_default.txt) and [here](https://raw.githubusercontent.com/MRtrix3/mrtrix3/master/share/mrtrix3/labelconvert/fs_a2009s.txt) respectively.
+
 !!! note
     The full list of output files can be found in [The ClinicA Processed Structure (CAPS) specifications](../CAPS/Specifications.md).