Add comment about trouble : character

tracim/tracim/lib/utils.py

     """
     ISO8601 current date, adapted to be used in filename (for
     webdav feature for example), with trouble-free characters.
+
     :return: current date as string
     """
+    # INFO - G.M - 19-03-2018 - As ':' is in transform_to_do_bdd method in
+    # webdav utils, it may cause trouble. So, it should be replaced to
+    # a character which will not change in bdd.
     return datetime.datetime.now().isoformat().replace(':', '.')
 
 class TracimEnforceHTTPS(BaseMiddleware):