Add explanation of the new csv file

docs/STEMMUS_SCOPE_on_CRIB.md

@@ -34,6 +34,14 @@
       provides parameter inputs for PROSPECT, leaf_biochemical, fluorescence,
       soil, canopy, aerodynamic, angles, photosynthetic temperature dependence
       functional parameters, etc.
+    - input_soilLayThick.csv (optional): A file to change the discretization of
+      the soil layers of the STEMMUS model. An example of this file is in the root
+      directory/example_data folder. This file should be copied into the InputPath
+      folder. If this file exists, it will override the default settings of the soil
+      layers. The file has three columns: 1) layer number, 2) layer thickness, and
+      3) maximum root depth. The user is free to change the values of the three columns.
+      Also, the number of rows determines the number of the soil layers and the total
+      thickness of the soil column (sum of soil layer thickness).
 
 2. Config file: it is a text file that sets the paths **required** by the
     model. For example, see [config_file_crib.txt](../config_file_crib.txt) in this
@@ -124,4 +132,4 @@ main script using MATLAB command line in a terminal:
 
 ```bash
 matlab -nodisplay -nosplash -nodesktop -r "run('STEMMUS_SCOPE.m');exit;"
-```
+```