Update documentation regarding naming of buffer files

Clarify how the path of the files are constructed and
what names should be used to avoid confusion.

README.md

@@ -139,4 +139,29 @@ terminology definitions should be followed:
 - start_pulse_id and stop_pulse_id (not end_pulse_id) is used to determine the 
 inclusive range (both start and stop pulse_id are included) of pulses.
 - detector_folder (root folder of the buffer for a specific detector on disk)
-- module_name (name of one module inside the detector_folder)
+- module_name (name of one module inside the detector_folder)
+- data_folder (folder where we group more buffer files based on pulse_id range)
+
+
+## Data location in buffer
+
+The written by sf_buffer are saved to:
+
+[detector_folder]/[module_name]/[data_folder]/[data_file].bin
+
+- detector_folder should always be passed as an absolute path.
+- module_name is usually composed like "M00", "M01".
+- data_folder and data_file are automatically calculated based on the 
+current pulse_id, FOLDER_MOD and FILE_MOD attributes.
+
+```c++
+// FOLDER_MOD = 100000
+int data_folder = (pulse_id % FOLDER_MOD) * FOLDER_MOD; 
+// FILE_MOD = 1000
+int data_file = (pulse_id % FILE_MOD) * FILE_MOD; 
+```
+
+FOLDER_MOD == 100000 means that each data_folder will contain data for 100000
+pulses, while FILE_MOD == 1000 means that each file inside the data_folder 
+will contain 1000 pulses. The total number of data_files in each data_folder 
+will therefore be **FILE\_MOD / FOLDER\_MOD = 100**.