filesystem: add comment to inform about checks in add_file()

The method add_file of class CardDF does some constraint checking
to the basic file parameters (e.g. fid). Since one might also expect
those checks in the superclass CardFile lets leave a comment to make
the code better understandable.

Change-Id: Iebae28909fe6aade3bd4024112a222819572d735
Related: OS#4963

pySim/filesystem.py

@@ -55,6 +55,10 @@ class CardFile(object):
             self.parent.add_file(self)
         self.shell_commands = []
 
+	# Note: the basic properties (fid, name, ect.) are verified when
+	# the file is attached to a parent file. See method add_file() in
+	# class Card DF
+
     def __str__(self):
         if self.name:
             return self.name