add example

the issue being the hd5 and zarr are planned for future derivatives, so the example uses something not currently supported

src/derivatives/introduction.md

@@ -138,6 +138,18 @@ containing BIDS paths at the level of the multidimensional file
 BIDS path rules MUST be applied as though these paths existed within the dataset.
 Metadata about the multidimensional array SHOULD be documented in the associated JSON sidecar file.
 
+Example of preprocessed data (here relmat indicates a 3D relational matrix in 4D node*node*time*frequency band):
+```Text
+└─ derivatives//
+   ├─ descriptions.tsv 
+   └─ sub-001/
+      └─ eeg/
+         ├─ sub-001_task-listening_desc-preproc_eeg.edf
+         ├─ sub-001_task-listening_desc-preproc_eeg.json
+         ├─ sub-001_task-listening_meas-crosscoherence_relamt.hd5 
+         └─ sub-001_task-listening_meas-crosscoherence_relamt.jon 
+```
+
 <!-- Link Definitions -->
 
 [definitions]: ../common-principles.md#definitions