Add comment

Author: James McKinney

csvkit/cli.py

@@ -68,6 +68,7 @@ def __init__(self, args=None, output_file=None):
         self._init_common_parser()
         self.add_arguments()
         self.args = self.argparser.parse_args(args)
+        # Output file is only set during testing.
         if output_file is None:
             self.output_file = sys.stdout
         else: