[FIX] clarify that scans.json is allowed and recommended

src/03-modality-agnostic-files.md

@@ -327,13 +327,14 @@ Template:
 ```Text
 sub-<label>/[ses-<label>/]
     sub-<label>[_ses-<label>]_scans.tsv
+    sub-<label>[_ses-<label>]_scans.json    
 ```
 
 Optional: Yes
 
 The purpose of this file is to describe timing and other properties of each
-imaging acquisition sequence (each run `.nii[.gz]` file) within one session.
-Each `.nii[.gz]` file should be described by at most one row.
+imaging acquisition sequence (each *run* file) within one session.
+Each neural recording file should be described by at most one row.
 Relative paths to files should be used under a compulsory `filename` header.
 If acquisition time is included it should be under `acq_time` header.
 Datetime should be expressed as described in [Units](./02-common-principles.md#units).
@@ -349,13 +350,16 @@ Additional fields can include external behavioral measures relevant to the
 scan.
 For example vigilance questionnaire score administered after a resting
 state scan.
+All such included additional fields SHOULD be documented in an accompanying
+`scans.json` file that describes these fields in detail
+(see [Tabular files](02-common-principles.md#tabular-files)).
 
-Example:
+Example `scans.tsv`:
 
 ```Text
-filename  acq_time
-func/sub-control01_task-nback_bold.nii.gz 1877-06-15T13:45:30
-func/sub-control01_task-motor_bold.nii.gz 1877-06-15T13:55:33
+filename    acq_time
+func/sub-control01_task-nback_bold.nii.gz    1877-06-15T13:45:30
+func/sub-control01_task-motor_bold.nii.gz    1877-06-15T13:55:33
 ```
 
 ## Code