Update 'readme.txt'

explanatory note on particular folder structure

readme.txt

@@ -16,6 +16,7 @@ TrialEvents.mat contains Trial events structure (see CellBase).
 StimEvents.mat contains Stimulus events structure (see CellBase).
 EVENTSPIKES and STIMSPIKES files contain pre-aligned spike times (see CellBase).
 The Tagging/<sessionID> folders contain tetrode recordings in optogenetic tagging sesssions in the same format.
+Please note that CellBase code relies on this particular folder structure (see CellBase documentation.)
 
 --- oo ---